You don't need any fancy tools -- pretty much what you'd guess:
- masking tape
- matte knife
- spray paint (primer, color, clear coat)
Here are a few before and after shots.
Step 1: Design the New Paintjob
I made a mock-up using Paint.net, which is free and fairly easy to use. I took a stock image of the phone, created layers for each color, and then tweaked the colors (hue/saturation) for each layer. You can find tutorials for this elsewhere. One of my results is below. Primitive but it kind of gives the idea.
Or you can just sketch it the old-fashioned way!
Now go to your local hobby shop or home despot and buy primer, colors, and clear satin or gloss finish. Spray paint is MUCH easier to use than brush-on paint. I used Tamiya and Model Masters. Be aware that some of the sparkly colors require a basecoat of a similar color -- read the can first.
Step 2: Disassemble Your Phone
The best way to find out how to take your phone apart properly is to find your phone's service manual, which are the official instructions, or just search for your phone and "disassembly" and you'll find others' instructions on how to do take your phone apart and put it back together.
That manual should also tell you exactly what tools you need -- possibly some silly screwdriver bits you don't have on hand.
Remember not to force anything. A lot of these pieces are snap-fit, which means you will have to pry them apart. Do it carefully. Follow instructions and don't lose any screws! You should end up with something like this.
Step 3: Mask the Phone
Then cover the phone in masking tape. If there are any straight lines on the phone, align the tape with their edges to leave those areas exposed -- then you won't have to trim it with your knife later.
With the phone mummified, trim the tape from the areas you want to have paint on. Lay your phone down flat, and hold the matte knife near the tip for maximum control.
Step 4: Paint
Paint cures best if you do this outside on a warm, sunny day.
Apply a single coat of primer, and follow the cans' instructions for how long to wait between coats of paint. Remember, apply thin coats or you will get drips. Don't rush or you will regret it!
Step 5: Reassemble
Step 6: Make a Custom Background
Go show off your "new" phone!