This is a no-fuss, success-guaranteed method.
1. Wait until your child is past 3 and shits like a man in his pants. (This step really sets the mood). Like really gross. Like think of your dad taking a shit. Now think of your dad taking a shit as a three-year-old. Disgusting right?
2. Wait until your man-shitting 3 year-old approaches you and says, "Me wear underwear." (Better this than the 8-year-old pants- shitter right?)
3. Put child in underwear. Preferably licensed character underwear.
4. Wait. Watch. Make child feel uncomfortable.
5. Reward child with awkward amount of presents upon completion of BM.
6. If child makes an accident on floor, put face in it. JOKING, DO NOT COMPLETE #6 UNLESS PANTS-SHITTER IS A DOG. In which case, there shouldn't be any pants involved.
7. Child should be using the toilet now. Child should also have an awkwardly high level of self esteem because of the weird praise you've provided for poop and pee.
AND THAT'S IT!