Sit in Nadanusandhasana, like so.
Roll up a doubled blanket to make a thick roll and sit on it, on your hunkers.
Make sure it is high enough and secure enough that you are steady. Fix it so that it is.
Place elbows on knees and rest head in hands. Fingers are on forehead or cupping sides and top of head and thumbs are close by ears.
Settle with eyes closed.
When you are ready, block both ears with the thumbs. Now you will make a steady humming sound, like a bee, not up and down, but along one low note, and listen to it reverberating in your head.
When one round is completed, keep thumbs in ears and take a breath or two. Then repeat about 7 to 9 more times. You can take breaths between rounds or do it continuously whichever suits you. Listen to the breath coming in and out when not humming. Listen intently to the sound of hum when you are humming.
When you are finished, keep the thumbs in ears and listen to the internal sounds. Continue in this manner for several minutes and when you want to finish, remove thumbs from ears, keep eyes closed and listen for a few moments to external sounds. Then come fully out.