We did it with DD in Feb and it worked! I too was doubtful but it worked (well, except for getting her to tell us when she had to poop but that came about 3 weeks later). As far as liquids, we had tons of variety from boxed juice, to water to mixing up some emergen-c (for kids). They want you to push water more than the rest (not much milk). We packed up the cloth diapers and she helped me put them in the garage saying bye bye baby diapers. The program has you stay by your child for those 3 days and you have to hold true- no distractions like internet, TV, phone, etc. The point is so you can see their cues and run (yes, literally run) to the bathroom and have them use the potty. If they have an accident you simply tell them pee or poo goes in the potty and you move on. Nothing negative. But when they do go in the potty, you praise them to no end- even if it’s only a bit after a partial accident. Make calls to relatives, have special toys (she got a new toy every evening), treats, anything to let them know they are a big boy/ girl and you are proud of them for going in the potty. You do have to say ‘Don’t forget to tell me when you have to go potty’ vs. Do you have to go potty? You want them to tell you. We followed the nighttime routine somewhat. DD is a beast to wake at night so we limited water intake before bed and woke her at 2am to go potty. We have night lights so no lights were turned on. It took a few days for her to understand just pee and we’ll go back to bed. Now she’ll wake during the night and I’ll take her potty and she’ll go right back to sleep. DD is my 1st child but I honestly think the 3 day method was perfect for us.
A friend did it with no underwear and it worked for her. We used underwear for DD and she was fine.