Step 1: Size up your existing antenna hookup
The connector coming off the existing internal antenna is going to have to be pushed back into the notch it comes out from. You'll quickly realize it doesn't go back far enough and obstructs the little metal flap from closing back. We'll cut part of that flap off.
Step 2: Eyeball the route for the cable to travel out of the battery compartment
Step 3: Cut a notch in the Airport Extreme flap/door to let the cable out of the Airport Extreme slot
The flap should close around the internal antenna's connector with it shoved all the way back into the notch it comes out from. Unfortunately, the flap probably won't stay put any longer because you just cut off the piece of metal that keeps it in place. But that's ok. Now you need to notch the battery to let the cable out.