#1.
http://forums.d2jsp.org/topic.php?t=3417311&p=17456588#p17456588 - This tutorial does eactly what you ask in easy to follow steps.
#2. Dont want the rectangle shapes? basically it's like step 3. Go to google and search 'shape brushes for cs(whatever version you use)' download whatever shapes or designs you are interested in and add them into photoshop, pull out your eraser and select the shape and apply at the edges to shape the signature. (you can also do this with a plain jane eraser)
#3. File > New. Set up you dimensions and at the drop box (backround contents) at the bottom select 'transparent'. Now say you'd like a pop out signature, make the background, ctrl and select all layers use, hit ctrl e, which combines them (make sure you're done and thats what you want) now throw the render in there and make it bigger than the actual backround, so that its on the white/gray boxed area, then save as a .png or .gif as said above and select 'interlaced'. There's a preview for this in my profile as #1. Quite simple really.