Step 1: Wait until s/he sits down on their own. Click, then treat.
Step 2: Repeat several times
Step 3: Say "Sit" as soon as s/he sits down. Click, then treat.
Step 4: Repeat several times
Step 5: Say "Sit!" and see if s/he will sit. If they do, click and give him/her a treat. If not, go back to step 4.
GOOD LUCK :)
p.s. Sorry for not putting this trick up yesterday. I lost track of time!