How do you use more than one sample in a song

I'm still a little new to producing, but it baffles me how people mix samples from different songs into one. Do they have to be in the same key? Around the same tempo? Do you warp them or something? So far, I've only been able to use one sample as a primary part throughout a song, but if someone can shed some light on how producers (or you yourself) mix more than one sample in a beat, it'd be greatly appreciated.