To catch grass carp you need a vegetarian based bait

I've heard you can catch grass carp with sweet corn and native fruits and vegetables but for the most part I have been told a doughball type bait that is made of grass or weeds.  Never really fished for them though, hear they are harder to catch than common carp.

Grass carp can spook really easily, so make sure they can't see you and don't cast right on top of them.  There are grass carp in a pond near my house and they are very hard to catch.  I've only caught a few; one on bread, two with worms, and one fly fishing.  I tried using dough balls, but they didn't work for me.  In my experience, you can only catch them when they are feeding on the top of the water.  good luck!