The classic Ring. This diagram can be found all over the place. I have two different links available from two sources but the end result is the same.

Difficulty : Beginner
Number of Bills : 1
Denomination Recommended : 1$ or $2
Credits : Money Origami and Origami Instructions.

Diagram 1 [PDF]
Diagram 2 [PDF]

Front Angle
Ring 01-01
Bottom Angle
Ring 01-02
Top
Ring 01-03