Combining two songs (medley)

[ttttrigg3r]

I was at a wedding when one of the performer played A Thousand Years by Christina Perri, then went into Payphone by Maroon 5, and then returned back to the first song.  I think it flowed perfectly.  I looked up the key signature for both songs, and they are both in the key of Bb Major.
Now I've not taken a Harmony class so I don't know too much about the theory behind it, but I'm interested in the rules and reasons when mashing two songs together.
For example I want to take Home Sweet Home by Motley Crue and Imagine by John Lennon to start.  They're both in the key of C. I want to start with Home sweet Home and transition into Imagine. How would I do that?