with a photo manipulation program
the best one to get is photoshop version cs2 or higher (it goes up to cs5) you can buy that in a store or you can consult a pirate down at your local bay.
The free alternative is GIMP, it works nearly just as well, but most will tell you to get photoshop.
after youve downloaded the program start reading tutorials, just get on google or deviantart.com and search "sig tut" or "gimp sig tut" if your using gimp
keep reading them and practicing, you should learn fast