I made my first collaboration on Threadless, for the 'Taking Leaps' challenge.
A collaboration is where you work together (collaborate) with another designer to make a t-shirt design. In my case, I worked together with MrHankey. I gave him an idea to work on, and he gave me one.
My idea was a design showing the good and evil side of Dexter (the serial killer, not the animated one). On one side his friends and family, on the other the murders, his "collection", and so on.
MrHankey's idea was to make a design around the pun "geologists rock". With several geologists striking rock-like poses, as if they were in an actual band. After giving each other some feedback the designs changed a bit from the original ideas, but I think it only improved them.