I'm pretty much a noob at this myself, but from what I've found you've got two main components: Monomer and catalyst.

With some monomers, the catalyst is oxygen or nitrogen, making them unsuitable for photoresins because they'll simply cure on contact with the air. Others use more exotic catalyst, like Magnesium or fluorine, etc. For these you can use a chemical that either contains one of these in a way that either passively lets it interact with the monomer, making a two-part casting resin, or one that must be energized to allow it to interact with the monomer, making a photoresin. In general, simple monomers are generated by mixing an oil, a gas and a precise heat.

The tough part of photoresins is figuring what monomer will work with what catalyst, and what catalysts are photoreactive. And then figuring out what oil and gas to get the monomer and some source for the catalyst. Once you have these three, it's just a matter of cooking and mixing (and hopefully not exploding or getting mistaken for a drug lab).

There's a discussion over in this thread about possibly making Linoleum photoresin from linseed oil and manganese vitamin suppliments.