Set it impression side up and allow it to set. Brush away any heavy dirt remaining, but don't try to thoroughly clean it at this point. Wrap the cast in newspaper to protect it during the trip home. The cast should be allowed to set up for a few days, giving it time to completely harden. Once set, you can clean off any remaining dirt with an old toothbrush.
Some collectors paint the plaster surrounding the actual track before displaying.
If you need help identifying the animal that made the track, check with your local library for a field guide. The casts you make this way are actually impressions of the feet of the animals that made the tracks. The side with the impression should of course be pointing up. Fill in any gaps around the edges with paraffin. Grease the first cast and frame well with petroleum jelly.