Here's a drawing of what I use to print above the pocket:
This is made out of a thin piece of plywood with lines drawn on it for centering the stencil and squaring it to the notch where the pocket will be placed. It's taped down onto the platen.
I pull the shirt down past the "notch" in the wood, then, holding the top corners of the pocket, pull the shirt into the notch, checking to make sure it's even left and right. Smooth the fabric laying above the pocket, and print.