I'd be glad to help.
I'm predicting you wish to install an already made kit, and not something custom? If so, then the process is quite manageable. First thing's is first, Remove all deliberating parts (which are pretty easy to access). Replace them with your new body components. Sand them down pretty well (I anticipate you know how the sanding process goes). Take the parts off again. base coat it with primer and pre-fit all parts;this is to balance out the gap spaces and make sure you're getting a proffesional look on all fittings. Remove parts again and take your kit to the pray booth, get the action going in there and once you're done, install all parts for the last time. If you want to go even further on the process, (making molds) I'd suggest you purchase a nice case of blending putty and hardner.
In cases of making custom bodykits. You must first construct your foam armature. Model upon that using industrial styled clay. Y2Klay works best, and once you have your shape, make a negative copy using Epoxy Gel (Tooling foam). Or you can pay a shop to fiberglass your design.
In your case of installing tailights, It's quite simple actually.
1.Open trunk to gain access to your old mounting points.
2.Unscrew your bolts and disattach all wiring.
3.Bring in your new set of tailights.
4.Connect your standard wires to the new lights.
5.Bolt them in place(Udjust them for correct fitting.
6. Close lid and wholla you're done.
Well, hope this helped you out a bit.