mostly it's just old-fashioned elbow grease as you have found out.

Leveling: There are a bunch of ways, but all sort of follow this:
String mason's line really, really tightly exactly 3 1/2" above the level that you want the sand. Level each line with a string level so that both are level and the same height.
Make sure your straight-edge is straight (You can look down the edge and see this). Now using the lines as guides, use the straight-edge to level the sand.

There are 88 bazillion ways to vary this method but they are all the same in principle.