If the ground is not totally, and I mean TOTALLY frozen underneath your rink, the water will simply absorb and you'll have a tough time building it up. I recommend putting down a tarp and making a border out of snow. Also, know that it's likely going to ruin the grass underneath, so go into it with that piece of knowledge.
Also, make sure that you're only putting down a skim of water at a time so that it can freeze quickly. Otherwise it will just make slush and that freezes all bumpy and is a bummer to skate on.
My kids would DIE for a rink in the backyard, but I'm not willing to sacrifice the grass. There are plenty of ponds/public rinks to skate on around here anyways.
Have fun!