start with an image that's well within the size of your brush. make sure when you create it, the background is transparent. you'll know because it shows up with checkers in the back. select the brush and color you want, then stamp it onto the image.
next you'll want to go to edit - cut, or it CTRL+X.
make a new image of the same size and use the paintbucket tool to fill it with the color you want the borders to be (I'm going to use yellow). now edit - paste... - as new selection or hit CTRL+E.
drop the brush you cut well within the center of your new image.
now go to selections - modify - expand. you can match your settings to mine, or choose your own border width. as the numbers go up, so does the border with. play around with it until you get something you like
your image should end up looking something like mine