Usually the first thing dog owners want to teach. It’s helpful in many situations and is a great exercise for impulse control.
Hold a treat in front of your dogs nose. Slowly (so they follow you) move it upwards and over their head. Once their bottom touches the floor, mark and then reward. When they’re reliably doing this, you can add you cue, i.e. ‘sit’ before they actually touch the floor. Mark and then reward.
Once they’ve done this a few times with the cue on, it’s time to try it without a treat in your hand. Give the verbal and hand signal, mark the behaviour and then get a reward right at the end.