What you need:
-blunt scissors or a blunt knife
I used 6x4 cm boxes with 1 cm in between each two. That way, you can fit eight letters on your two pieces of paper. Make sure you leave a few cm at the bottom.
You can align all the boxes, or for a fun effect, make them swing from left to right.
-each letter is IN MIRROR VIEW! because you will later turn the page around
-each letter entirely fills a box, that way they'll all be the same size
-each letter is pretty thick, otherwise it will be too fragile
Make letters like Y, I and P that stand on one "leg" a little wider at the bottom, so that they'll stand stable once cut out.
With a ruler and blunt scissors or a blunt knife, trace along the lines where you'll be folding. Those are the horizontal lines at the base of a letter, and the lines you just traced in the middle between those "base lines".
Now, fold the lines along a ruler. Fold the paper up at the base lines, then turn the paper around and fold the other lines up on the other side.
If this all sounds too complicated, just look at the pictures how it's supposed to turn out.
You can leave the card as it is, classy white, or add some colour.
For another original birthday card, check out my other instructable: