To my knowledge there are 2 easy ways to make borders and this is how
First make a new document and clikc ctrl-a and the whole sig shuld be outlined2. then go to image on the tab and clikc stroke(under fill) and it should open a window
l3. put in setting 4px is good and choose color and tada!
another way is 2 steps
1. Make a sig right click on it on the bottom left box thing hwere the layers are (right click) and go to blending options.
2. look at pic
and yer done