Here's how I did it:
1. Choose which washi tapes wanted and what the message will say. (Originally I had picked out pastels, so some of my photos are using pastels.) Write out the message on graph (or regular) paper to make it easier when applying the tape to the wall.
2. Use a level to draw a light guideline at eye level.
3. Make small crosses on the wall, without sticking them down completely, so there are enough of each color to complete a letter. For instance, 12 crosses are needed for "H". (I found it easiest to put one piece of tape straight up and down followed by the second piece straight across instead of trying to make crosses are a 45% angle.)
4. Start peeling off the pre-made crosses, turn then 45%, and stick them to the wall, adhering them completely. Use the level to make sure the lines are straight as you go. It's fine if they aren't perfect.
5. Keep making crosses and letters until your message is completed. Erase the guide line and smile.
Then if you're me, you realize that you can't see a few of the letters because they're too light. Sigh, and then go back and replace the practically see through letters. Then enjoy. I kind of still prefer the pastel color pallet, but it makes the message easier to read with the brighter colors. And a few of the pastel tapes weren't sticking very well, so it all worked out in the end.
I buy my washi tape from Etsy, Joann's, Michael's, and even Target. It's become very popular since I bought my first basic set of 10 colors back in 2009. There are so many fun patterns and colors.
If you end up doing this project, leave me a link in my comments, email me, or tweet to me. I's love to see!