Touching the right knee to the floor before the Blessed Sacrament.
Find the tabernacle first. It is where the Blessed Sacrament is reserved, usually marked by a candle or lamp burning nearby.
Face it, bring the right knee all the way to the floor, pause for a beat, and rise.
Do it when you enter the church and when you leave, and any time you cross in front of the tabernacle.
If a knee will not take it, a deep bow from the waist is the accepted substitute and always has been.
The knee goes down because someone is there. The gesture is a confession of faith made with the body before the mouth says anything: this is not bread, this is the Lord. Catholics believe the whole person prays, not just the mind, and the knee is the part of the body least able to pretend.
Bending the knee was a court gesture long before it was a church one. In the Roman world you went down on one knee before an emperor, and Christians who refused to do it for Caesar did it freely for Christ.
By the Middle Ages the gesture had attached itself specifically to the Real Presence, as Eucharistic devotion grew and tabernacles moved to a fixed and visible place in the church.
The present rule is simple and old: a genuflection is for the Blessed Sacrament and for the Cross on Good Friday. Everything else gets a bow.
Genuflecting toward the tabernacle when you enter and leave is the normal expectation, and the Church names a bow as the equivalent for anyone who cannot kneel. No one should feel watched over this. A reverent bow made well is worth more than a hurried knee.