As the grout dries on the surface of the shards, brush it off with an old toothbrush. Work inwards from the edge, taking care not to dislodge grout from between the pieces of crockery. Cleaning off grout at this stage will save a lot of work later, although set grout can be cleaned off with sandpaper.
Allow the grout to set overnight, but don't put it in a warm place as slow drying will give the piece more strength. Go over it again with a brush and then a soft cloth to remove the remaining haze and dust of grout.