To make a texture mat for a model you would first want to start with getting all the textures you want to use for that model and convert them all to the same format.
I use bitmap for my preliminary textures so that i can edit them with MSpaint using the cut and paste method.
Make sure to start with an even number of pixels for the heighth and widtn...Ie...1024X1024,512X512,256X256....etc.
Then cut and paste each texture you are using on to the mat so that you have one mat for the whole model.
When you create texture mats that are perfectly square like this you can resize the mat to a smaller size without losing the original UVW mapping placements.
You will spend more time actually creating and editing the texture than you will applying the mapping to your model.
Hope this helps explain how single image texture mats are created a little better.
3D modeler for C&C Source
www.cnc-source.com