Stand between parallel bars. Squat down and grasp parallel bars from above. Kick legs up inverting and balancing body upside down. Legs can be kept bent or straight, positioned vertically.
Raise body up a high as possible by pulling shoulders toward ears. Lower body to original position and repeat.
Falling from an inverted position or hitting head on structural bar when flipping body may result in very serious injury. Adequate clearance must be allowed for head to clear any obstruction during both inversion and dismount. Exercise can also be performed on gymnastic rings. A safer position would be hanging centimeters above well padded flooring. Since this movement becomes more difficult as full shoulder elevation is achieved, height criteria for shoulder elevation may be needed. For example, raising shoulders until slope of shoulders becomes horizontal may be considered adequate depending upon individual body structure and range of motion with lighter weight. Also see ROM Criteria.
There are very few practical ways to make this exercise easier except placing lower legs on higher bar above.
For greater resistance, have partner stand off to side, hold weight on rear end. Only strongest will be able to perform full rep(s) with exercise partner sitting on back of thighs seated in opposite direction.