With support of of your hands sit only on your buttocks

First thing to cultivate sensitivity in the buttocks.

This can be started by firts feeling your left hand while lying down and feel only your hand.

Give all your attention to the hand after all your attention begin to focus on the hand then begin to fell what is happening in the hand and begin to note any sensation, any jerks or any slight movements that are happening hand.

Once you become good at this

The in a seated position apply te same to the buttocks.

The idea is not transfer weight of the body from one buttocks to another instead cultivate a feeling of sensiivity and through this sensitivity let the weight of the body gets equally distributed and let a feeling of centering arise.

Once this centering arise it would throw it to your navel center and you would be centered inside you would forget the buttocks, you would forget the body, you would be thrown to the inner center.