Joining afghan strips together

Hello everyone. I am looking for the best way to join the five strips of my baby afghan together. I have used garter stitch on the edges that will be joined. I was intending to use the mattress stitch but i think that only works with stockinette. Thanks in advance.

http://knitty.com/ISSUEspring04/mattress.html
Here’s a tut showing how to mattress stitch garter st. Hope this helps.