First off the grout should not be runny. Secondly the float should be held at angle so that it acts almost as a squeege. If you go diagonally across the tiles you should be able to get most of the grout in to the seams. I would wipe a lot of the excess up with a dry towel than wait for it to create a slight film and sponge off with a medium dry sponge. Lots of luck. I personally think it is a tough job.