Print out the shape you want to emboss, straight edges are easier and more effective.
You need some card, 200gsm+ is probably ok, just think cardboard or cereal box.
Overlay the print out and cut out the design.
Place the cutout on the piece of paper you want to emboss, in the position you want it. Then holding it in place, flip the piece of paper over and start rubbing the edges lightly with the tip of your bone folder or spoon or peeler or whatever it is your using. Try and get the edges as neat as possible.