DO not try this unless the guitar you are using is one you dont care much about. Dont do this to your main guitar, or one that has alot of value. Save that for the pros. This will explain some basics to customizing the body of your guitar, by yourself. You need to be able to paint, in whatever way you want to do it. Some use spraypaint, and some use brushes or tape in my case. I think the best approach is the AirBrush that they use to paint tshirts with.
First of all, remove your strings. Once you have done this, it may be wise to go ahead and clean your pickups, and pickguard. Some people may frown upon this but I use windex and a clean sock =) it works quite well.
Once you have a clean pickguard, go ahead and remove the screws around the outside. dont mess with the screws around the pickups( I only play fender guitars so other guitars may be different) because they raise and lower your pickups. If you see a pickup moving up or down as you screw, thats means you are on the wrong screw. Remove all the screws until you can raise your whole pickguard out of the body. (note that there will be wires still attached that will not allow you to completely remove the pickguard(with pickups still attached) from the body.
OK. On almost every electric guitar, you can unscrew the input jack. The wires from the input jack will be what is holding your pickup guard and pickups in the guitar. On stratocasters at least. On my guitar the wires ran through a hole in the body, and it kinda leaves you in a messed up position for trying to take the pickguard out.
This is the part that is going to be risky, so be very very careful. You need to clip the wires that are connected to the input from the pickups. Only do this though if you cant go ahead and pull the input out of the body also. If you have a stratocaster this is for you though.
After you have finally completed the task of taking the pickups and input out of the guitar, you should have an empty guitar body connected to the fretboard. That is easy to take off, just flip the guitar over and remove the four screws under the fretboard, and it should come right off. now you have the guitar body, with no wiring. This is what you want.
Now, you can sand the body. I am assuming that the guitar you are trying to paint has a messed up paintjob, but if you have a guitar with a beautiful paintjob it will work also, its just not gonna be that pretty when you get done unless you are a good painter. Make sure you sand it good, try to get all paint off, as best as you can.
Now for the fun part!!! I used blue tape and covered my guitar with the tape in random places.
Then i used white spray paint and sprayed it until i covered the entire guitar in white.
I carefully pulled back the blue tape to reveal my mastarpiece(:
ROCK OUT! c(: or just listen to iTunes..... ya, that one(: