Here's a new explanation - maybe this one is going to be a bit clearer
First, the iphone goes in through the bracer.
The hole for the screen is smaller than the actual iphone (so the only the screen and a part of the Home button shows)... so that's why it goes in from underneath.
So here are the steps... once you have both parts (computer and bracer) made
1- You'll have to carve away some of the foam from the bottom of the computer part so that it fits at the right angle when it sits on the bracer.
You have to be careful when carving away the foam - the bracer is usually not a cylinder (meaning that you have the same diameter on both side)... it should look more like a truncated cone. This means that for the computer part to be "leveled" you should carve out less foam from the side of the computer part that is closer to your wrist (and more from the one that is closer to your elbow).
2- Once you've craved the foam from the computer part - you should have a clear idea where the computer will sit on the bracer. You should mark the spot where the screen line up on the bracer.
3- Once this spot is marked - you can cut it out of the bracer.
This is going to be the hole in the bracer you'll squeeze your iphone through.
Make sure it's big enough for the iphone, but not too big... the computer part of the pipboy should be able to cover this hole completely once it's glued to the bracer.
4- Do a test fit... so the screens from the computer part lines up with the hole you've just cut in the bracer... and if everything looks right - glue the 2 pieces together with hot glue.