Well other than stating the obvious response that border less signatures don't have borders:
Open up your canvas
Make something that doesn't extend all the way to the sides of the canvas or cut the edges with an appropriate eraser brush
Save the file as a .PNG (this will allow transparency)
In all honesty though, I would recommend to stay away from border less signatures. Work on your basics, learn the techniques and concepts, and then, IF you still have this random urge to do pop outs - try them.