Decide where you want your zipper to go on your shirt. Then mark a line under the zipper.

Carefully cut along the line you've just drawn. Make sure you don't cut through the back of the shirt.

Place the zipper inside the shirt under the opening. Fold the edges of the opening and pin them to the zipper. You don't have to do the top and bottom just yet, because those are difficult and you won't need to sew those yet.

I had some trouble with the top and bottom. I decided to cut a little further than the zipper goes and the make two cuts to the side so I could easily fold the edges. I later sewed those edges onto the rainbow fabric.

Now sow the zipper on. If you're sewing it with a sewingmachine, I suggest using a zipperfoot.

Now get your strip of fabric. Make sure it's about 3-4 inches longer than your zipper. I only added 2 inches for seam allowence on mine, but that wasn't enough, because I accidently streched the shirt a little bit when sewing, so I didn't have enough left over at the bottom to make a nice hem.

Turn your shirt inside out and pin the strip on your shirt, including a nice double folded hem. In the picture I also pinned the top and the bottom already, but don't do that, because of what I explained in step 6.

Now sow the strip on. I handsewed the top and bottom, because they're tricky.

And there you go. A super cool zipper, rainbow shirt.