(UPDATE: I created a tutorial on using textures with a gel layer! http://www.conceptart.org/forums/sho...d.php?t=127825 )
Open Your paper pallete
First find a Texture you would like to include in your art and then open it in painter. Also make sure you have Use our wiki to find texture resources. Click Here for Wiki
Secondly select the entire canvas of the texture(make sure you are on the correct layer).
Third Under the paper tab, click the arrow > , Than click Capture Paper.
Name your File then Adjust the Crossfade. The Cross fade is how the Pattern will mesh when the paper overlaps its edges in large canvas.
Select the small thumbnail to cycle through the list of collected papers.
Select a brush with the option for GRAIN, not all brushes have this option. The lower the GRAIN option is, the higher "roughness" of the grain will show against your brush strokes.
The sliders in the Paper Palette are:
Try playing with these sliders to experiment with different effects.
Here is a list of brush types that have the GRAIN option. Other categories have brushes options such as the brush in the PHOTO category called ADD GRAIN, i personally shy away from this brush since its a little unwieldy to control.
Hope this helps!