Slow and steady. I weed eat around trees and shrubs so that it's easier to use the ride on mower. I've seen the results of grass cut with a weed eater. If the grass was low to begin with, the result looks pretty good, keeping a low even cut , try not to cut the ground. If the grass is high, 6-8 inches, the the cut grass will be on top and look untidy. The thing to do then is rake up the cut grass, remove it, and weed eat again. This grass may even need a third pass depending on length. You can leave some grass behind to act as mulch which is good. But because of the long grass length to begin with the final result will look a bit uneven.