Just remember to apply a little past the edge of the design on the decal and trim it up close before dipping in water, otherwise if there are ragged edges, it tends to bunch up in a lump.
One coat is usually enough - try to avoid multiple passes over an area especially once it starts to dry, otherwise lumps and brush marks are a possibility. IF you feel the need to recoat, let it dry for about half an hour before doing so.