1) Wait until she is staring down her third birthday, stays dry most nights, asks for underpants, and can pee on demand whenever you put her on the potty before a bath.
2) Buy a package of Pull Ups, and then decide you don't want to use them yet because your other two-year-old can't even say "potty" much less use it, and you really just want to endure this potty training misery one more time. Toddler is undeterred. She will insist on using them in the following order: all of the Mike and Sulley Pull Ups, followed by all of the Minnie Mouse Pull Ups. When she gets to the Ariel Pull Ups, she will turn up her nose and ask for underwear again. Agree with her-- Ariel is a brat.
3) Take her to Target with the intention of buying underwear. On the way to the toddler underwear, ask her what kind she wants. "Frozen," she says. There are no Frozen underwear. She does not want Tangled underwear. She does not want Disney Princess underwear. She does not want Monsters, Inc underwear from the boys section.
4) Get on eBay and pay $25, plus shipping, for seven pairs of Frozen underwear.
5) When the underwear arrive, hide them. You still aren't in the mental place to do this potty training thing again.
6) Older sister finds the underwear and wants them for herself. Tell her they are for your toddler, and you are not, under any circumstances, ordering more from eBay. Older sister throws them at your toddler in a huff.
7) Tell your toddler that she can wear the underwear under one condition: she cannot pee in them. If she pees on Olaf, he will melt. If she pees on Elsa, she might get her booty frozen. Start saving money for eventual therapy.
8) Toddler chooses to wear the Anna underpants.
9) Proceed with normal life. Occasionally remind toddler to go sit on the potty. After three days, she has had two accidents. On the third day, she tells you when she needs to poop.
10) Gloat over your child's genius and precocity. Pat yourself on the back-- this time it worked just like all of those "potty train your child in one day" books said it would. Or, rather, shake your head in amazement that this has been so easy. This is your fifth time on the potty train, and the first time it was relatively pain-free. She may sleep like a newborn and have the left hook of a prizefighter, but she's a dream to toilet train.