I always find for me, I don't decide to put a joke anywhere. I start building a premise and as I do it I just let funny things pop into my head. This then branches into other funny changes, and the whole thing kinda starts shifting and molding into this whole new thing that makes me laugh my butt off LOL. I don't really... decide to be funny. I try to be open to what is funny around me.
But humor is strange, cause it's based on wavelengths, and not everyone is on the same wavelength, so not everyone is struck by the same comedic elements.