This isn't limited to knowing how to obtain sketchy bathwater testosterone (affectionate) either. ((Though do know how to do that without getting into legal trouble like I did oops)) It's not too difficult to learn what tests your endo orders for you to monitor your levels and how to read those results. The girlies have it a bit harder than those of us that usually only need T to get things in working order, but none of that is inaccessible knowledge. Knowing your levels can help you make decisions about dosages and which meds you should be on.
And know how/where to get needles if your insurance doesn't want to pay for them or the pharmacy is out of stock. Needle exchanges are wonderful places period but can also help you out here. Barrels are accessible without a prescription, I switched my pharmacy issued 3ml barrels with 1ml barrels to make measuring dosages and injecting easier.
Know HOW to administer different types of meds. SubQ and IM injections for hormones are, assuming you don't have a too large amount of fluid to inject, essentially interchangeable. Be able to do both, even better if you can help others who can't self inject with their shots. Again, this information is more for the girlies, but know all the different routes of administration (buccal, sublingual, oral, patches, up your butt, etc.) and how that effects medication absorption.
Know how to work with the side effects of other parts of medical transition that cis doctors tend to not know or warn you about. Vaginal atrophy can be remedied with topical estrogen cream, and you can figure out your own ways to source that. How to combat hairloss if that is a thing you so desire. How to combat acne. Deal with T dick sensitivity. Can't start T yet but periods give you dysphoria? Hop on the depo shot. Want facial hair? Try these things.
Even if the knowledge won't directly help you at any point its good to keep as a backup. And who knows, maybe you'll pass that knowledge on to somebody whose life will be saved because of it.
Lean on your community, this is knowledge we have and pass on as a collective. Most of the people I know who DIY to a certain extent do so with other people's scripts, usually when there is an issue having their own script renewed or filled in time. (ALWAYS refill as soon as you can with everything you can and if you're hardcore know how much you can skimp on a dose or occasionally skip to build a stockpile up for yourself or others) DIY communities tend to not advertise for obvious reasons, but if you get yourself connected with your other trans its not hard to seek these places out by word of mouth.