Find the picture you want to have huge on your wall. For me, it was no contest - mmm, Amanda Palmer. I also deliberately chose a black and white picture because I gather it's easier to do it with that, especially if you don't have a great printer (like me).
and upload your image. You can choose to crop it and decide how many sheets you want it over. I decided to make mine 25 sheets, so 5x5 in portrait. Generally the bigger you make it the better it looks. The instructions on there are really good.
Print off all your pages but make sure you keep them in order - they should work from left to right going down, as though each page were a word in a book (if that makes sense). So for me, it went
1, 2, 3, 4, 5
6, 7, 8, 9, 10
11, 12, 13, 14, 15
And then cut off the borders (unless you were clever and removed them)
While you're being very clever and keeping them in perfect order, stick them up on the wall as I said. A lot of people use wallpaper paste to get a permanent look, but as I'm in halls I just used white tack. Depends how permanent you want it.
Step back and admire your work. Why not, take some pictures of yourself next to it! Here you can see the scale of it. It's pretty big =)