If you are editing the entity file (FPE) and your source model is already textured then place the textures the model uses in the same folder as the model file and in the FPE field 'TEXTURED', you can either specify your texture by name or leave it blank. Leaving it blank will force the model to load in and use the texture it already has, which if the model is okay will look textured when in the game. My advise is trial and error. There is no hard and fast way to get your models into FPSC, and no official tutorial which takes you through the steps at the level of detail you might need, but keep tinkering and once you have it, you will not forget it. There will be many users who have already made progress in this area, so I would go as far as you can and then ask specific questions to overcome obstacles. As we get closer to V1, this issue will be taken as a matter of increasing importance as this is something many users will want to do out of the box.
"We are the knights who say...eki eki eki fatang loopzoing, zanziga....ni"