Even for people in design school it can be hard for people to mix patterns. I can still hear my Mom's voice telling me not to mix stripes with plaids. Don't listen to my Mom (or maybe even your Mom)mixing patterns isn't as hard as it seems. I like to think of it as layering "boy" prints with "girl" prints. The boys always wants to be around the girl don't they? Remember when Joey was in love with Rachael? Joey is bold and stands out with his "How you doin'"?Rachael was more girly and soft. That's the same kind of idea.
Ok got that idea in your head? Good , ok...well first of all they have to get along. Meaning the colors should work well together and not compete...cough cough Monica and Ross.
Next one should be bolder (boy) and the other more delicate(girl). The patterns should not be the same size. One stronger and straight the other curvy. Keep the scale while mixing those patterns. White or black negative space in the patterns helps give the eye a rest.When in doubt lay on on top of each other and make sure they don't give you a headache.
Let's look at an example:
Notice how these two are the perfect mix? Same color family, mixed sizes, not competing and perfect! Now go out there and makes "Friends" with patterns *clap clap clap*