I think you split it down into fairly small clumps, maybe 3 or 4 plantlets. Someone else may be able to comment more on that.
Anyway, the easiest way by far to plant hairgrass, and other plants for that matter, is with a pair of tweezers! The tweezers enable you to get the plant into the substrate and pull out without disturbing it too much, so the plant stays put. With fingers too much substrate is moved and it gets difficult (and frustrating!)