Quoting from this post ^^
"-10 minutes of stomach vacuums. super easy. stand in front of a mirror to make sure you're doing it right. the goal is to pull your belly button to your back, you pretty much just suck in your stomach. you will know if you're doing it right because your transverse abdominal muscles (the deepest layer of ab muscle that's on the outer part of your belly) will feel tense. try and hold this for a whole minute, it's hard!
-5 minutes of seated contractions. sit high and tight in a chair, and pull your belly button all the way back to your spine. hold it there for 2 seconds, then let it out halfway. do this for 5 minutes or you can break it up into intervals."