You should start off with thinking about the design that you want to paint on your guitar. A stencil always comes in handy whenever you are trying to print something down. Therefore, pick a stencil and start drawing whatever you think you want to paint on your guitar.
Now you must know the guitar parts that you need to paint. Make sure that the strings are not attached to the guitar since you have to paint the whole body. Make sure that each and everything that you have to paint is fully covered. Moreover, remove all hardware which is attached to your guitar so that it does not catch the paint that you want to apply on your instrument.
Once you are done with painting your guitar, leave it as it is and at least wait for one or two days so that the paint has dried off completely.
Acrylic paints are usually preferred. They are the ones which never remain wet and are the best to use on top of wood. You should always let the paint dry for at least four hours after the first coat and then four hours more once you are switching between different colours.
Once your guitar is fully dried and you are sure that it will not mix up different paints if your finger touches it, it is time for you to apply polyurethane on top of it. Always use the instruction written upon the package as it will help you apply the liquid just the way it should be applied. In case you want extra durability, take the guitar to a body shop and it will really you clear the coat the way you want it. Just make sure that you have not attached your strings to the guitar and you have removed all the hardware from it.