Do these on a stationary trainer. Place short stools, chairs, milk crates, or small children on both sides of your bike. Unclip one leg and rest it on the small child and pedal with just the other leg (I am available for baby sitting during my Tuesday single leg pedaling drills). Keep the cadence, resistance, and duration low for these until you develop your technique. Alternate legs about every 10-20 seconds at first and gradually (over several weeks of practicing) increase the duration, cadence, and then the resistance.
Pedaling with one leg will force you to move the pedal in full circles. You will notice right away how much work it is to pull through the bottom of the pedal stroke and lift the pedal back up and over the top. You’ll really feel these in your hip flexors (the muscles responsible for lifting your upper leg toward you chest). Try and eliminate the “dead spots” at the bottom and top of the pedaling circle, and keep the pedaling motion as even and smooth as possible. This will seem difficult at first, but you should begin to see some improvement after a few weeks.
Again, focus on keeping the legs relaxed and smoothing out the transition from one direction of pedaling movement to another. As with the leg speed drills, you should listen for the consistent, steady whirling sound that the tire makes on the trainers roller.
You can practice this on the road to some degree as well. Just focus on favoring one leg at a time. Pull through the bottom, un-weight the pedal as it’s coming up, and push your foot up over the top. Be sure to give each leg equal time. Try doing 3-5 revolutions with one leg then alternate to the other leg.