when i started, i made mapping a high priority and for this i made dummy textures for each individual piece. with trial and error figuring out the shape of the joint on my texture. basically i used the texture template and made colored strokes where i thought it might hit the joint.
Then i took a look ingame and afterwards adjusted it.
later i just took parts of older sets i made as reference to see. then also trial and error to fit it my needs.
hope this helps^^