First: Paint your store bought (don't dare to make one) letter dark brown (I used Apple Barrel's Burnt Umber)
Second: Take an old rag and rub black shoe polish on the edges of the letter
Third: Hang it on the wall to admire.
Enjoy your new work of art! And if you really do want to make your own letter out of cardboard, you can, because I did it, and I only 12 (it just takes a lot of patience)