First you go out and buy some cardboard letters. I bought mine at Xenos (dutch store) but you can get them at many places now.
Then you get your stanleyknife and start chopping. Ok, maybe not chopping but i'm not the best at working precisely so you can do it easy peasy!
Poke holes it the front about as big as your lights are. (I used a screwdriver that was the same size)
Someone asked me which lights I used, these ones. Bought them at IKEA for 2,99.
Paint it whatever colour you want. I used black for the sides and white on the front.
Looks horrible doesn't it?! Don't worry you won't see a thing later.
Put the lights in the holes. My fine motor skills are not that developed like I said earlier so this was HARD for me...
Turn around and turn the lights on, voila! Your very own letter light for under 10 euros.