Glass: This is probably the hardest part of resin kit builds unless you're an absolute pro model builder and the kit has perfect glass. Neither of those applies to me - TRUST me.
Here's how I do it:
1) Carefully and slowly, trim the glass until if fits the opening fairly well with no buckles. Don't cut TOO much or you'll end up with a gap - if you do though, it CAN be fairly well repaired - as long as you didn't go too far.
2) Tape the glass into place with a small piece of tape you can easily remove later.
3) Using a craft glue such as Modge-Podge and with a toothpick, lay in a bead of glue along the edge of the glass.
4) Fill any minor gaps with the glue. It will dry clear, and once dry - can be cleaned up with water!
5) Once dry, remove the tape and fill that area just like in #3 above.
Waiting for the glue to dry for the clean up and coating with Future!
When I'm totally done with my build, I use Future floor wax to coat the car - including the glass. It helps seal it in, helps hide the gaps, and gives the glass a thinner look.
As an FYI - I have never used more than one piece of glass for any of my builds - even those I've thought I've goofed on. The above methods can almost always mask the problems quite satisfactorily.