There is no need to spend money on garland for a birthday or any other holiday. If you have paper, marker, yarn/string, and a hole puncher, you are ready to make your own.
1) Make sure your triangles (or any other shape you choose for your letters - circles and rectangles look nice, too) are the same size by making the first one and then using it as a template to cut the rest. You will also want to make sure your holes are punched the same distance from the top of your shape, so begin by punching the first one and then lay it on top of the next one as you use it as a guide to punch the rest.
2) Write your letters on each cut out paper shape using a thick marker or stickers.
3) String your garland. If you like, you can use a little tape or glue to anchor the tops to one another. Now attach it wherever you like. I chose to hang ours like a swagging valance above the window between the curtain panels, using the curtain rod to tie each end.