This is a tutorial on how to paint with 3D models. If you do not know how to use a model I would recommend learning how first. First, get your 3D model in position on it's own layer.
Then outline it (I would recommend outlining in pencil, it gives it a feeling of acrylic).
DO NOT make the brush big, small would work better.
Then start painting over it, don't add details quite yet. (Make a new layer, then, turn the pencil layer to a low opacity. I painted with the Gouache brush and Oil paint. I used Oil paint for tints in the painting. )
Then start adding your details. I would recommend using Oil paint for the details.