Hide your pattern layer by clicking the EYE off.
Switch to your channels pallete , Create a new channel and mask out the part of the drawing you want to apply the pattern to. (Paint the area white)
If you see only black, make sure you click the "EYE" on for the RGB channel. Your new Alpha channel should then be 50% red so you can see what your painting over.
©2003 Patrick Shettlesworth