I know my jokes aren't the best, but I get a lot of them just by thinking about phrases, and turning the context on it's head.
For instance, the phrase 'I'm going to fill your face in' meaning I'm going to give you a good hiding. Who else might 'fill your face in', but without hitting you ? Maybe to do with eating, but also to do with plastic surgery. So, to turn it on its head, I made the gag -
'Plastic surgeon has been threatening me over money. He's told me to pay my bill or I won't get my face filled in'.