Sit in a comfortable position and relax the entire body. While breathing normally, rapidly contract and relax the anal muscles, trying to confine the contraction only to that area. (It is natural, however, that other muscles will respond. This will improve with practise.) When you feel comfortable with this rapid contraction and relaxation, you can slow down the practice so that you contract the anus upon inhalation and release upon exhalation. Be sensible and do not overstrain. Do not practise if you suffer from anal fistula.
Also note that when the body is tense and anxious we often unconsciously hold the muscles in the buttocks, anus and pelvic region quite tight. Check this out right now and if you find tension, release it. Check back regularly. Ashwini mudra is very helpful for releasing this unconscious ‘up-tightness’.