POCKETS?!?

[cameronangel.livejournal.com]

Hello, again! So with the help of a fellow craftgrrl, I have decided on a design for the bag I'm going to make. I already have all the supplies I will need, along with a general idea of how to put the bag together. I know how I'm going to attach the straps, put a zippered pocket on the inside, and in the flap, but I cannot for the life of my figure out how to put an open pock on the outside. Ya know, it's sewn on three sides and open on the top? Something like a cell phone pocket? I mean, I don't know how to sew a smaller square onto the larger one....
like this

I mean...I would normally put right sides together, sew the seams and then turn inside out, but that will not work here....so how can I achieve this look? I don't know where to begin!
Thanks, I promise to post a pic of the complete bag!

-Di-

(Edit: ok...so I did make a mistake, but I really don't know how to fix it for next time....I waited until the end to sew on the bottom. and of course, the bag has sections, so I cant just turn it inside out and sew right sides together (well I could on 3 sides). So I have to hand sew the last part. Pics tomorrow, after I hand sew the last part and get the straps.)