This is an in-depth tutorial for an assignment that I created for the DS106 assignment bank. I did want to put a disclaimer that I use photoshop for this assignment and I know that not everybody has it. As a possible photo editing tool, you could try PIXLR. It looks like it has some very similar basic functions that photoshop provides like layers and such.
Before & After:
The video below is a step-by-step guide with commentary on how to get this done in Photoshop. There are many video tutorials on YouTube that describe and teach many different ways of getting to the same effect. I highly encourage you all to browse around because not all methods work well for all people.