Hi Shadrac
You need to make yourself a box-joint jig (also called finger joint)
this one is similar to the oak-park type:
http://www.woodworkersworkshop.com/c...s/26509_files/
If you need help troubleshooting, or figuring it out, let me know through a Private Message and I will help you as best as I can. You really need to do a lot of trial runs with things like this to fine tune your work. The more you play with your router the better you will see all it's options and applications. I use mine almost daily.
~Julie~