Lay out your letters on a flat surface. It's best to put them against a straight edge of some sort. I used a scale.
Once you get them spaced exactly the way you want them, get your straight edge out of the way and put a piece of masking tape over top of it. This will hold the letters exactly how you layed them out.
Next, turn over the tape so you can remove the backing off of the letters.
Now you can tape it up where ever you want to put the letters. Push down on the letters firmly and them you can remove the tape and VIOLA! Your done.