I got this idea from Kelly Lee Housewifery. She has a tutorial there on how to make these so I won't go over it all here. I decided to make things a tad more difficult though and got really large cardboard letters. That means it required a lot more christmas lights and something to stabilize the letters so they didn't fall over or flop all over the place. To remedy this I went to Home Depot and got clear plastic wall corner protectors. I got 4 of the smaller ones rather than the full size ones. I over lapped them a little and it worked really well. I just hot glued them to the bottom and top edge like in the image below.
I also used one long strand of lights as opposed to small 20-light strands. I missed out on the holiday light selections so the longer strand had to do. It worked out well with the larger letters though, because I had the plastic corner protectors to reenforce everything.
When you put a dab of glue on the lights when inserting them, be careful not to get glue on the bulb. It can burn it out.