Grab a weighted barbell: doing a regular bicep curl-
1- 1st 7 Reps: go from the bottom of the movement up to the half way point (arms stop at 90 degrees and hands at elbow level).
2- 2nd 7 Reps: go from the halfway point up to the top of the bicep curl (hands near shoulder level).
3- 3rd 7 Reps: start at the bottom of the movement and complete a full range of movement all the way up (a full bicep curl with no stopping at halfway).
That is one set and how you do a 21s.