How to install basement retrofit windows?

I am installing retro-fit basement windows inside the metal casing frame from the old egress windows. Can someone tell me the best way to do this? How much gap should I have on the sides? Should I use shims and the foam insulation or should I just leave an 1/8 gap on the sides and use silicone caulk and put windows in the opening? Should I use a couple of screws through the vinyl window into the metal frame? I also was wondering about adding 3/4 thick solid vinyl strip at the top to drop the window down a bit as the metal casing frame is about the same level as the drywalled ceiling. Any suggestions?

 

Looks like you are on your own on this one-do what looks
easy- caulk where necessary--maybe some trim-you will be
fine.............