Just because you can paint a guitar, doesn't mean you should. If you're happy with your guitar except for the way it looks, you can safely remove the paint without damaging the guitar. As long as you work slowly and give yourself a lot of time, you can get your guitar looking exactly the way you want.
Step 1 - Clean and Prepare
Before you remove paint from your guitar, remove the hardware to get it out of the way. This means unscrewing the bridge, removing the strings, the neck if possible, and anything else on your guitar body. You then must make sure your guitar body is clean, so wipe it with a damp cloth and let it dry.
Step 2 - Use Heat Gun
Many guitarists swear by using a heat gun instead of chemicals to strip the paint. The heat gun gives you more control and it saves you from having to inhale nasty chemicals. But the heat gun is not without its hazards so make sure you read the heat gun's instructions carefully before beginning.
Do not place your guitar on anything flammable, such as paper, as the heat gun will ignite it. Test the gun first, away from your guitar. Then start slowly and apply heat to the paint you want to remove from the guitar. You will see it bubble up when it is ready to be scraped off. Make sure you put the gun on a low setting and work slowly. If you are in a rush and start on too high a heat, you could burn the guitar body. Also if you rush the job you may burn yourself, so be careful.
Step 3 - Scrape
Once all of the paint on your guitar has bubbled, turn the heat gun off and start scraping it with your putty knife or scraping tool. Some guitars may have multiple layers of paint and finish so depending on the state of your guitar, you may have to scrape harder than you think or go back to the heat gun after scraping off the first layer.
Step 4 - Sand
If you find that there are more layers of finish underneath the paint you've stripped, you may want to use sandpaper instead of the heat gun again if you're nervous about charring the guitar body. Start with a rough grade sandpaper and go finer the closer you get to the body.
If you don't have access to a heat gun or simply do not want to use one, you can skip Step 2 and go right to the sanding. This will still work, but might take an entire day to complete.
Step 5 - Finish
Even with scraping and sanding, you still might find some finish and paint on your guitar. In that case, you can either switch to scrubbing with a Brillo pad or you can use white spirits, a low toxicity solvent, that will get rid of any little pieces of paint still clinging to curves and nooks of your guitar.