The Roman Missal describes a genuflection simply: “274. A genuflection, made by bending the right knee to the ground …”.
Here is my more involved description and suggestions for practice. Step forward with the left leg. Bend the right knee to the ground, level with the left heel. Try to keep you back straight and head up. Hold the position for a few seconds. Then stand up by stepping back with the left leg, and straightening the right leg.
Having mastered the above, practice genuflecting with hands joined. This is described in the Ceremonial of Bishops: “107. Unless the bishop is holding the pastoral staff, he keeps his hands joined: [footnote 80: “Hands joined” means: “Holding the palms sideward and together before the breast, with the right thumb crossed over the left” (Caeremoniale Episcoporum, ed. 1886, I, XIX, 1).] when, vested, he walks in procession for the celebration of a liturgy, when he is kneeling at prayer; when he moves from altar to chair or from chair to altar; when the liturgical books prescribe joined hands.” (Reference: Ceremonial of Bishops, Liturgical Press, 1989, ISBN 0-8146-1818-9, page 43.)
The original Latin description includes that the fingers are to be extended, so they should be straight, when joined.
Sometimes you will not be able to step forward to genuflect. For example when saying the Creed on the feast of the Assumption and Christmas a genuflection is required at the words “and became man”. But there may be another seat in front of you. So in this case, move the right leg backwards a step, then bend the right knee to the ground, next to the left heel. Stand up by bringing the right leg forward.