First: Make a new layer.
Second: Double click on the stock layer to "unlock" it.
Third: Move the stock layer up.
Fourth: Use the pen tool, make your path, check the option, there should be a box, put 1 or something. It'll blur your stock so it'll fit better in your sig. Right click on path then "Make selection". Take the select tool , right click, reverse.
Press "Delete" , you should have your render.

Hope it's clear...!