I'm not an expert, but have been cutting for a few years now, so this is just from my experience:
You will either be a corner helper (in one of the back corners by the herd) or doing turnback up in front of the herd. If you are a corner helper, once the cutter goes into the herd and brings some cows out, your job is to get the rest of the cows that they are not using pushed back to the wall. Try and do this as quickly as possible without getting in the way of the cutter. You're also responsible for making sure the herd stays against the wall in the center of the pen. If they get too off centered, you and the corner helper on the other side will need to push them over until they are centered again.
If you are doing turn back, make sure you aren't putting too much pressure on the cows so that the cutter cannot get a good work in. You want to put enough pressure so that the cow is moving around, but not so much that it gets too panicked and flees. You will have to judge the cows individually, as some are slower paced and can take more pressure without getting too squirly, but others need a much larger "bubble". Try to keep the cow between you and the cutter, without letting it run past you towards the other end of the arena.
I will add any more I think of, hopefully some more experienced turn back riders can add something!