To do this yourself, simply: Hide something, leave clues, and make a treasure map.
All you really need is:
You might also want, but don't necessarily need:
I put 4 clues into white envelopes and taped them in the locations pointed out on the map (this was for a 4 yr. old, so we kept it really simple). Once it was time to hunt, I told everybody to put on their eye patch (for effect) and handed them the map, instructing them to find the clues and read them to find out where the treasure was hidden.
I made sure to make it clear that the treasure they needed to find was the birthday girl's presents and she needed everybody's help to find them!
Our map led from the 'START' words to a corner in the fence where they found the first clue; it said, "Look in another corner for your next clue." Then, "Look under something blue that you can walk up." Then, "Go to a girl's room to find another clue." Finally, "Look inside a box that is old and is outside to find the treasure."