Rough Timeline

Summer before Senior Year:

Start thinking about the programs you will want to apply to. What subfield are you most interested in pursuing? What kinds of things do you want in a program? Does location matter a lot or a little to you? Do some googling of astro programs and the faculty at those programs. Check out some of their publications on ADS. Check out the websites of their current and former grad students to get an idea of the kinds of things they're working on (grad students sometimes have more detailed websites than faculty).

Faculty at your undergrad institution can be a good resource here. If you have a research advisor, a course professor you vibed with, or perhaps the director of the undergrad program, or several of those. Ask to meet with them and discuss what they know about good programs to consider applying to for the science you are interested in, the reputation of the program, and what they know about various faculty in the program as mentors. Ask about graduates of the program - do they produce a lot of graduates who go on to win prize post-doc fellowships or have distinguished careers? Do grad students seem happy there?

You could even reach out to current grad students of specific faculty you're interested in and ask how they like working with them as a mentor, the culture of the research group, and the culture of the grad students and grad/faculty relations. I DO NOT recommend sending a generic email to lots of people asking for general advice. Target *a few* specific people and ask *specific* questions. Especially if you've met them at a conference or on social media previously. Grad students will be more frank and give you a better idea of what it's like to be a grad student there than faculty will.

I'm not sure there's much utility to reaching out to faculty you're interested in. 1 - they're pretty busy and are likely to not even respond to your email, especially if it's generic and doesn't ask specific questions (like "I wonder if you have and advice about applying". Don't do that.) 2 - They honestly will probably forget you. It's not personal, they meet a lot of people and are busy. Unless you meet them in person at a conference or smething (and even then...), they will probably not remember your name. 3 - Unless they're on the admission committee, they likely can't sway admissions even if they wanted too. So there is often little they can do to give you a leg up even if they want to hire you.

August/September:

Time to get serious. Narrow down the list of the programs you will apply to. Make a spreadsheet. You can use my template. Just make a copy into your own google drive and adapt to your needs.

Schedule GRE exams. Many astro programs are dropping the Physics GRE (huzzah!) but not all, so you may still need to take it if one of your programs requires it. (2 of the 7 programs I applied to needed it! Darn.) Most (but not all) will require the General GRE. Astro departments typically have full control over if they want to take the physics GRE or not, but usually the grad college is the one requiring the general so that's a tougher battle for them to fight even if they want to get rid of it. So keep careful track of which programs require what.
Here is a list of US and Canadian Physics and Astronomy Departments, their GRE reqs and application fees, links to policy, and some statistical info:

Apply for the GRFP. The NSF Graduate Research Fellowship Program offers three years of funding and a host of other benefits for your grad program. You should apply for the GRFP. You get two chances to apply - once as a senior undergrad and once before the end of your 2nd year as a grad student. Apply now otherwise you'll only get once chance! Add it to your apps tracker spreadsheet. Check out my resources page for more info.

Look for other grad student fellowships. Here are a bunch.

Make a website and CV. You want to be sure you are easily googlable. If someone googles Your Name + Astronomy, you should be the first result. You do not have to make a crazy website like this! At the very least it should have your contact info, a link to your CV, and a short bio including research interests. GitHub offers one free webhosting per user, and there are very nice html templates out there that are easy to set up and use quickly. I use HTML5UP, also Carrd.co has great one-page splash page styles.

CV != resume. Academia expects CVs, so do some reading on how to make a CV. Check out the CVs of lots of faculty and see what they have on there. And keep it updated! As soon as you do anything (present are conference, give a talk, win an award) update it immediately. Overleaf has some great latex CV templates.

Go to the AAS Winter Meeting. Abstract submission dates are usually around this time, so talk to your advisor about sending you to the AAS Winter Meeting in early January. Grad admissions folks will often seek out applicants at this meeting to have a chat. Presenting a poster at AAS gives you a chance to shine to the folks on the admissions committee reading your application. Ask your advisor if they can fund you. Also typically universities will have travel grants you can apply for.

Start writing personal statements/ statement of purpose.

October/November/December/January:

GRFP application deadline is usually in October. Pay special attention to the deadline because it is different for different disciplines. Most grad apps are due late November/early December. By mid-December you can sit back and admire your hard work. Attend the AAS Winter Meeting and present a poster or talk in January.

February/March:

By early- to mid-February you should begin to hear from programs you applied to.

Rejections: Sorry, it happens. Just remember it's not personal, there are any number of reasons why you weren't given an offer even if you had a great application. They could have had 40 amazing applications and only 20 slots to give out! Perhaps your research insterest were not a good fit and they want to give them slot to someone with a better chance of accepting the offer. Remember this is a match making process where you are trying to find the best program for you and they are trying to find the best students to succeed in their program. For example, I was rejected from one school because I wanted to work in direct imaging but the folks doing direct imaging weren't taking students that year. It would have been a waste of a slot for them to offer me one. So yes it stings to get a rejection, but it doesn't mean you didn't have a good application, and it doesn't mean you don't belong in astronomy! In this field rejections are the name of the game!

Offers: Congrats!! You received an offer of admission to one or more program you applied for. Nice! What happens next?

Visits! Well this is the fun part. The dynamic has now switched from you begging for an offer to them begging you to accept! Prepare to be wined and dined. You should recieve an offer outlining the pay and benefits, with information about the formal prospective grad student visit. Programs that give you an offer will plan a time for admitted students to visit and put on a program for you to experience their grad program, chat with grad students and faculty, and help you decide to go there. While specifics will vary, this will typically consist of a flight out to the university, two days (usually thursday and friday) to meet with faculty and grad students, and a third day of excursions to experience the town. For example, at Arizona we invite students out typically around the 2nd of 3rd week in March. Fly them in Wednesday night; Thursday and Friday they have group overview presentations on things like the details of getting a PhD there and DEI/Outreach, and they have one-on-one meetings with faculty relevant to their interests. Grad-student only dinners both nights. Then Saturday our program director hosts a brunch in his home, then students head off to several optional acitivites including a trip to the MMT (telescope) or a hike in the mountains with other grad students, followed by a party at a grad student's home that evening. Prospective students typically stay in the home of a current grad student, so you have plenty of access to pepper them with questions and to see what it's like living in the town as a grad student there. This is the time to ask any and all of the questions. There are multiple times where it's just you and the grad students, no faculty around. This is on purpose so you can ask the spicy questions and get the real low down on things. This is a big decision (your next 5-6 years!) so it's important to be fully informed.

April 15th:

This is the nationally agreed-upon decision date for grad school offers. By April 15th you should have either accepted or declined all the offers you recieve.

Highly recommend accepting or declining as soon as you've made a decision. If by mid-March you pretty much know what you're gonna do go ahead and do it don't wait til April 15th. This lets the program know what to expect and if you decline it frees up a spot that can go to someone on the wait list asap.

Beyond!

Congrats! You're going to grad school! Start thinking about the moving process and where you're gonna live. Reach out to current grad students for advice on where to live in town. Ask if the program offers moving support (but don't be too surprised if they don't). You can ask your potentional advisor if they can pay you if you want to get started in the summer. BUT! Take a break man. Seriously. You've been working hard at undergrad and grad apps, take some time to decompress and have fun. You have 5-6 years of grad school work, don't be too anxious to get started. It's a marathon not a sprint!

Remember it is also possible to defer starting for a year. So if you accept an offer but want to take a gap year, chat with them about deferring and starting the next year. This happens often, it's not abnormal. If you're a traditional student who has gone all the way through school without a break, consider a gap year. In my opinion it's a great idea for traditional students.

Lots of advice from other people!

Don't take my word for it.

Let me know if you have a link of valuable resources, I'm eager to collect more! Send me an email!