I’m not the guy to ask about it overall (I’ve still never done it end to end), but the theory might give you an idea of what’s involved.
Essentially, you’ll eventually need a silk screen that blocks paint from going thru in certain areas (where the shirt should stay white on a white t-shirt). I would guess we have some blank silk screens somewhere.
The normal way this is done is coating a silk screen in UV activated resin and expose it in a dark room etc.
Some people just create a mask on the vinyl cutter for this same purpose (blocking paint flow). It’s not as durable, but it’s a bit easier (depending on skill sets) and should hold up for small runs.