1. Decide on the antenna you want to use.
2. Based on it's mount requirements, choose the type mount that matches the antennas base connection and how/where you want to mount it.
3. On the mount look for cable length and connector at the scanner end (if it's not BNC, you'll have to get an adapter, maybe two).
A few antennas are designed to not need a ground plane, but for the others that do body metal (or other) being your ground plane will play into where you decide to mount it.
NMO mounts can be magnet mount, trunk/lip mount, permanent mount (drilling a hole in body metal), bumper mount, mirror mount (maybe a couple others that I can't think of now.
Consider routing of the cable through the vehicle to where you want it to be in the vehicle, where the scanner is going to be.