Rob,
Daryl has highlighted the basics of the drill above -- a 20-footer from opposite sides of the hole.
That said, a few additional insights...
The Goal:
-- hit 10 putts in a row that don't violate the rules.
-- said putts are rolled 3 one direction, 3 the other direction, 3 the original direction, and 1 final putt back.
-- any time a putt violates one of the rules, your counter is set back to -0-.
The Rules:
-- counters are either made putts, or putts that stop in the Safety Zone.
-- the Safety Zone is defined by a 34" semi-circle (approx. a putter length) that starts @ back edge of the cup.
-- hence, putts that don't get past back edge of cup, or are more than 34" away from hole in any direction, don't count.
Thoughts:
-- in the beginning it's OK (if not a good idea) to start with a flat putt that has minimal to no break.
-- an important part this drill is building pressure, the first few times you make it to putt #8-9-10 you will feel some!
What Happens:
-- you will quickly lose your focus on the hole, which is good.
-- as feel develops and you become adept at the drill, putts will start going in a rate that will amaze you!
Later:
-- pick a 12-15 footer with some break, be sure it's the same amount of break both directions.
-- add a rule <--> any putt that crosses the straight-line, in front of the hole, is a violation (even if it ends in Safety Zone).
Have fun!