1/ record your voice saying something
2/ play the recording in reverse and imitate it (this can take a couple of tries)
3/ then record yourself speaking backwards
It's more fun if in step 3 you do something that would seem weird in reverse, like pouring a glass.
Then you play that video in reverse, and you have something strange: you saying things in a weird but intelligible voice, while the liquid goes back to the bottle.
My coworker says what sounds like "Rih koff ra dom" and then the candidate plays it back. In the middle of our pretty crowded office, during a weird silent lull, this candidates phone just yells out "Motherfucker!"
For some reason that coworker had memorized how to say motherfucker backwards. He must have been waiting for that moment for a long time because it was perfect (if not quite work appropriate) timing.
Hilariously, she would sing the line in reverse when live on stage [2] as well. (No doubt she memorized the line from the studio recording.)
All credit goes to Mark Mothersbaugh