Should I measure it before or after I crush it? Recipie calls for 1/2 cup crushed corn flakes (but I am probably only making a 1/2 batch so I would need 1/4 cup)

Crush then measure.

We have a cookie recipe that calles for crushed cornflakes and I have always crushed before I measure. Other wise you will end up with half the amount since the big flakes take up lots of room.

I'd do it exactly as TexasSugar said.

If the recipe calls for "1 c. crushed corn flakes," you crush first. If the recipe calls for "1 c. corn flakes, crushed" you measure first.