The tree top finish is one of the best low risk finishes for the single leg. This same finish can go by many names (high outside finish, foot sweep finish, etc.. take your pick), but the result is the same.
Once in on the single, pop the ankle under your armpit keeping control over your opponents foot. You should have it so tight that the only way he will get loose is if the shoe comes off. It has become more and more common to see athletes counter this move by doing this:
It works, quite often, at the High School level. Don't let it happen to you. After securely controlling your opponent's foot in your armpit lean back slightly *(ever so slightly), and push your forearm into his calf muscle. The pressure he was putting on the leg under your control will be shifted back to his free leg, making it much easier to "Tree Top" him.
The next clip shows Matt Demaray executing a perfect tree top single finish in the NCAA finals against Troy Sunderland of Penn State. Matt was a 2x NCAA Champion and one of the best college wrestlers to have ever competed.