Okay, so you got the height figured out, the cap drilled, now let's glue or teflon the connections together. The whole thing is not just glued with PVC Cement... there are some parts you'll want to be removable so you just make those connections snug with teflon tape. The pic below should clearly illustrate which connections are glued and which are tefloned (is that a word ). I used a rubber mallet to tap the cemented connections.

9 - How do builld a standpipe for your Internal Overflow