1. Use 1 ball
2. Putt 9 times from 4-12 feet around 1-3 practice holes with relatively flat, straight putts alternating distances.
Level I Goal: Sink at least 5 putts while having the missed putts finish within the length of the grip on your putter.
Level II Goal: Repeat Level I and increase severity by adding slopes
Level III Goal: Repeat Level I with 6 sinks
Level IV Goal: Repeat Level II difficulty with 6 sinks
Level V Goal: Increase your distances to 4-15 feet
Level VI Goal: Increase the # of sinks and level of difficulty
Why 9 putts? Start tracking the distances of your first putts on your pin sheet when you play. You will see that you have these distances about 9 times during a round.
I encourage you to do some version of this several times during each practice session. Build your Library of Confidence to resource when you need it during a competitive round.
Next time you have doubt in competition when you face these putts, remind yourself that you know how to sink putts!