This is a topic that I get asked about all the time. I have a LOT of potty training experience, between my own three little monsters, the preschool classes I used to teach, the daycares I worked at, plus providing ABA for kids with autism for over 10 years. There are very few kids I haven't been able to teach how to use the potty. So, while it may not be a glamorous thing to be an expert on, it certainly does come in handy!
First things first...the child in question has to be ready. There is no magic age, and it is different for all kids.
Signs to Look For
~ Your child remains dry for at least an hour when in diapers.
~ They let you know when they need to be changed, at least some of the time.
~ They can pull their own pants up and down (doesn't have to be perfect, but they should be able to manage mostly independently).
~ They can follow simple instructions (come here, sit down) and imitate other people.
~ They express an interest in what you are doing in the bathroom, or want to sit on the potty.
You really need ALL of these criteria first, in order to be successful in a reasonable amount of time. If you're not seeing something, like being able to pull their pants up, ask yourself, "have I ever let them TRY?" Sometimes it's not that they aren't capable of doing something....they just haven't been given the chance! Same thing for being interested in what Mommy and Daddy are doing in the bathroom...do you let them come in with you? Most moms do, but if you're potty training a boy especially, it's helpful to have dad take them once in a while, too. Or older siblings, cousins, etc.
The best way to start is with imitation. Put their little potty in the bathroom, and when you go to the bathroom, they sit on their potty across from you. Don't ditch the diapers yet; this is just to get them used to the routine. They should go through the whole process with you, from pulling their pants down to washing their hands after (and there are very few kids who don't LOVE to wash their hands!). Have anyone else in the house (who is willing/comfortable) do the same thing. The more opportunities to practice, the better!
Next, pick a good time to start. You'll need at least a couple of days where you don't have much of anything else planned, and you can just stay home. Also, don't pick times when you have some other stressful events going on in your life, or your child's life. New siblings arriving, moving, starting a new daycare, mom going back to work, etc...all bad times to start potty training. Aim for an uneventful weekend.
Then, start getting your kiddo excited about it! Take them with you to the store and pick out new undies (you will need A LOT, at least 8 pairs, more if you don't want to do laundry every day). They should be loose fitting...not so loose that they fall down, but loose enough that they are easy to get on and off. Read potty stories, watch potty videos, it doesn't really matter which ones, there are zillions of them.
Finally, the big day is here! Dress your child in just a t-shirt and underwear. Then have them go potty, starting with pulling their own undies down. Let them sit for 1-2 minutes, or as long as they can stand it. I liked to keep a stack of books in the bathroom to keep them occupied while sitting. Make it fun! You don't want them to dread the next trip to the bathroom. And don't make it too long. A couple of minutes is plenty to start with. On the off chance they are successful on the first time sitting, make a big deal about it! Kids love the potty song and dance (it really doesn't matter what version you do; make something up and have fun with it!). Stickers are also a big hit usually. An M&M or something else small is fine for a reward, too. It's easy to stop giving them the rewards when going potty becomes a habit.
Most likely, though, they're not going to do anything the first time. So, praise them for trying and sitting nicely, have them pull their pants back up, wash their hands, and set a timer for 15 minutes. (If they are successful, make it 30).
Then, this is VERY IMPORTANT!
In between trips to the potty, periodically ask them if their pants are still dry.
Remember, there are two goals with potty training:
First, of course, is to get them peeing and pooping on the toilet. That one is pretty obvious.
But secondly, and possibly more importantly, is for them to NOT pee or poop in their pants any more!
There are a lot of kids who will happily pee every time someone sticks them on the potty, but they will also happily pee wherever and whenever! That is hardly successful potty training!
So, you ask, "Are your pants still dry?" Show them how to check (and double check, because sometimes they really aren't sure, and sometimes they lie). Then praise them, a LOT if they are! "Wow, you are such a big girl/boy! I'm so proud of you for staying dry, etc!"
The other reason this is important is that THIS is their motivation to STAY dry. You want them, eventually, to be telling you when they need to go. It takes some time for kids to start recognizing that signal sometimes, because odds are, they've never had to pay much attention to it before. It also keeps the potty idea in their mind, without having to spend the whole day sitting there.
After that, it's basically just repeat as necessary until they start to actually use the potty and stop having accidents. This can take anywhere from half a day to several weeks...just stick with it, be consistent, and try not to get discouraged. They will get it eventually.
So, what do you do when accidents happen?
First of all, be prepared, there WILL be accidents. If you can't handle cleaning up puddles, well, I suggest just waiting until they're old enough to say, "What the heck am I still doing in diaper?" (It does happen; that's the approach a relative of mine used with her oldest, and what do you know? He was potty trained in one day. Of course, he was 4 1/2.....)
So be prepared! Put down towels if you don't want your couch to be peed on. Buy a bottle of Nature's Miracle from the pet store for your carpet. And most importantly, RELAX! Do NOT freak out at your child! Accidents are just that: ACCIDENTAL! Remember that potty training is a learning process, and is all completely new to your baby!
When accidents happen, you say, "Uh oh! We don't pee in our pants! We need to go to the potty!" Then, have your child take off their wet pants and put them in whatever laundry basket you have designated for that job (or right in the washer, which is my preference!). Have them help clean up any messes. This is another built in motivator for not having accidents! No one wants to clean that up! At the same time, though, it's not a punishment. It's a natural consequence. You make a mess, you clean it up! As long as you are not yelling or freaking out, it's not punishment.
So, they clean up the mess, and then go back and sit on the potty again, while you remind them, "The potty is where you go pee (or poop, or whatever terms you want to use)!" Have them go get clean undies and put them on. Then it's back to repeating as necessary.
Gradually, your child will have more successes than misses, and you can slowly increase the time between potty trips.
So...what happens if you do all of that, and you're still not having any luck, and you and your child are getting fed up with this whole process?
Back off! Sometimes even when kids meet all the signs and can follow through with the routine, they aren't actually ready. So you can STOP! Before it becomes a huge source of frustration and turns into a daily power struggle. And that's FINE! In fact, with my second, she was 100% completely day time potty trained at 21 months...for about 2 months. And then she started having tantrums over getting her potty seat dirty. Yes, really. She would also freak out about having wet panties. AND, she was so tiny, that even with the ring insert on the toilet, she was in danger of falling in (or, at least SHE thought she was in danger, so she wouldn't sit there). So even though she'd been in underwear for months, I put her back in diapers. She was mad the first couple of days, but still less upset than she had been about all the other issues. A few months later, when I was off work for Christmas break and she was bigger and 26 months old, we tackled it again with the ring seat, and TA-DA! Back to using the potty in less than two days, with no screaming or frustration from either of us! Never had a problem again! So, even though she met all of the physical and developmental milestones that showed she was "ready," and even though she had every capability of being successfully potty trained, emotionally, she just wasn't there yet. When she was *really* ready, it was remarkably easy!