I don't know how much or little you already know, so I'm going to stream of consciousness infodump what I know. also as a disclaimer, I've only been on DIY HRT for about 4 months. if anyone else sees me say something wrong or has suggestions for improvements, I implore you to call it out

Sourcing medication

A big list of vendors can be found at https://hrtcafe.net/Estrogen/estradiol_injections.html but off the top of my head, the big homebrew vendors that ship internationally include open gate labs, panacea, otokonoko, astrovials, voix celeste, and dragon ordnance. there are also some that sell pharmaceutical industry product, like inhouse pharmacy, but their prices are expensive. personally I buy from open gate labs since they ship within the US, which means it doesn't go through customs. they also do third party testing, which gives me a little extra peace of mind.

Choosing an ester

Injectable estrogen is available in different esters, the main difference among them being half life, which affects how long it lasts in the body before you need another dose. estradiol valerate is the most commonly prescribed, but has a short half life, to the point where some people find they have to inject twice a week. estradiol enanthate is most common among DIYers (including myself), and lasts long enough that you only need to inject every 7 days. other esters like cypionate and undecylate supposedly last even longer, but aren't as well studied, so results may vary.

Dosing

Dosing should be determined by mass, not volume, since different products are compounded at different concentrations. as an example, if the vial you are using says it is compounded at 40mg/mL, and you want to take a 6mg dose, that's 6/40 = 0.15mL. If you used one compounded at 20mg/mL, you would take 0.3mL to get the 6mg dose.

A dose between 4-8mg/week is recommended for estradiol enanthate. Start on the low end at first (4mg/week) and increase if blood tests are low. The dose and frequency will be different with other esters.

Drawing and injecting

This is the sort of thing that's best described visually, so instead I'll recommend this video. One thing I'll point out is that he's doing an intramuscular (IM) injection, while some people do subcutaneous (subq) injections, which uses a shorter needle, and involves pinching the skin.

So as far as injection supplies go, you will need:

rubbing alcohol pads
band aids
luer lock syringes (1mL)
≥1 inch ≥25 gauge injection needle
≥25 gauge drawing needle (do not try to go bigger (lower gauge number) than this, or the risk of damaging the vial stopper goes way up)
sharps container to store used needles (you can buy a "real" one, but you can also just rinse out a small laundry detergent bottle and use that as long as it can be securely closed and is it reasonably good condition)

if you're doing subq, you can use a shorter injecting needle, or substitute the needles/syringe with a ≅1/4 inch 30 gauge fixed insulin needle.

if you're using the luer lock syringe/needles, one tip I'll offer is to draw the plunger out a bit more before switching needles (at the point where it's out of the vial), so any liquid in the hub of the needle (space between the tip of the syringe and the actual needle) isn't wasted. after switching, make sure to push the "excess" liquid back into the new needle hub before injecting (look for a tiny drop at the end of the new needle) so you're not injecting air.

Storage

Store vials in a clean, dry, dark, place. Make sure to inspect vials before use, and do not use if you see particles floating in the liquid, if the liquid is cloudy or discolored, or if there are any noticeable leaks when turning it upside down. I recommend having at least 1 spare unopened vial at all times if you can afford it. An unopened vial that has been properly stored should still be good for at least 3 years.

Payments

pretty much all DIY HRT vendors have to get paid under the table, which means they mostly only take cryptocurrency. bitcoin is widely accepted, but some also take litecoin, ethereum, xmr, and others. I don't really have a crypto exchange I'd recommend, paypal screwed me by only letting me withdraw like a month after I bought (lol), so I bought more from coinbase, which provides an easier customer experience with low fees, but then they had a data leak like a month later (lol). IRL crypto ATMs tend to give you a really shitty exchange rate. This is the future of money . One thing I'll point out is that if you really don't want to go through crypto, dragon ordnance does take bank transfers through a service called wise, but I've never used it, so idk if it's good.

Where my knowledge gets murky 😬

Anti-androgens

anti-androgens are often available from DIY suppliers as well, but I've never used them (I've always been on monotherapy, even before DIY). theoretically if your estradiol levels are consistently high enough, T production slows down to almost nothing.

Progesterone

never used this either, but FWIW you probably don't need to know anything about it at this point, since I always see people say it's best to start once you've been on estradiol for at least some months.