Well, I will answer the silly question first... nope, you can't cross cannabis with watermelon. Not even with its cousin hops, although you could GRAFT onto hops if you were REALLY, REALLY bored.

The process for crossing 2 strains is this:
Grow some strains
Think about what you like in each
Pick parents that show those traits
Cross them
Keep records
Continue to cross offspring until the offspring are stable for the traits you like.

It's complicated and time consuming, but that's the simple answer to your question.