how do you grease a line to float?

Depends on how much line you need to put the flotant on. Easiest is to put some of the stuff on a paper towel or clean rag, strip off however much line you want to treat, and reel the line back on through the rag or PT. It just needs to coat the line, not soak into it, no sense wasting it, and it comes in handy every now and then.

candle wax will float line just run the line through the wax or use a line floatant like mentioned above.

What kind of floatent do you use?

Fly line, you can get it in any fly fishing section of any tackle shop, including BPS or Cabela's. Way back in the day my dad used to use a few drops of dish soap mixed in water, but I could never seem to get the amount right to have it work well. YMMV. Just curious, have you tried actually using grass (like actual lawn clippings) for bait for your grass carp? Used to fish a small pond on a golf course, the grass carp would wait until after they were done mowing and then just graze along the edge eating the grass that blew into the pond. Tied it on the hook with a small rubber band, used a small float to help with casting weight.