I started when my son was 15 months old and was no longer waking up with a wet diaper. I would just sit him on his little potty once in a while and if I had to go I'd bring him with me, or my husband would and we'd show him the pee pee or poo in the potty, and let him flush it. It sounds gross letting your kid see your pee/poop, but it showed him that the potty is where it goes. I started putting him on it and letting him go around naked a little while every day so that if I "saw" he had to go I could take him quickly, or it was easier to get on the potty than having to take off everything. He was completely pee trained when we moved from Korea back to the states, and stopped going in the potty completely (22 months old). We got him back to going pee and he was pooping in it, (29 months old) then I had my daughter. My husband was off work for a month, and he did fine, but when he went back to work he started going on the floor, I assume for attention. And still does it on occasion, now at 3 years 2 months, but 99% of the time he goes in the potty. He's been completely potty trained again since a couple months before my daughter was born in Dec, but since then he's had a few accidents/on purpose for attention. Anything that changes her daily life may throw her off...us moving did for Elijah. It's confusing, it's new, it's not what they're used to...and with a new baby there's that and, shared attention, not getting as much attention, a new baby that EVERYONE talks about and wants to see first. EVERYONE asks about the new baby, "how old is he/she" I tried to include my son as much as possible...when someone would say how old is the baby I'd ask which one...Of course they'd say the little one, but some people caught on and would talk to Elijah too, others would just say "she's so cute" after I told them and walk away. I think regression is normal for them, or for them to act out when there is a new baby. It just takes time...I did special things with Elijah and made it a point to tell him that only HE could do it because HE was a big boy, only HE could pee/poop in the potty...baby sister was TOO SMALL. Make it sound like she's special because she gets to and the baby doesn't. It'll keep her interest more, and she'll be happy she gets to do something and the baby doesn't. To begin with I also used treats, like an M&M or something when Elijah went in the potty. It would make him want to again.
My 3 older children were all completely potty trained by 2 years old. It does take time and alot of patience but they will learn. Anytime she eats or drinks wait about 20 mins and take her to the potty. Turn on the shower or sink and sit there. I mean it could take 10 mins to an hour, but you do it until. Keep some books or magazines handy. I did.
Make a HUGE deal about it when she does use the potty. And if she has an accident until she knows it's wrong to go on herself just clean her up and put her on the potty so she understands that, that is where she needs to go. She is going to have alot of mistakes. I say do not use pull ups because they are to much like a diaper. Buy her some underwear and tell her how pretty they are. My DD was the hardest one to get potty trained. But she turned 2 in march of 09' and by Sept. of 08' she was sleeping in underwear. and completely potty trained.
It's hard and frustrating but when she is trained you will be sooooo proud of your self.