How and when should I teach my toddler manners?

I agree with all the above comments. I started with day one for both. Since the older one knows what it's expected, the second one learns from her. My kids are very often the only kids in a group that use their manners. My 2 year old son gets upset when someone doesn't say you're welcome after he's said thank you. They both also say excuse me and God bless you. My daughter is starting to ask "may i please" instead if "can i".. It's important to teach them their manners so they don't turn out to be some slouchy poor mannered children. Elbows of the table, finish all your food on you're plate, do not ask for seconds if you know you might not finish it,.. All the etiquette when eatingat someone else's house..

My friends son will demand things.. Even from strangers. If he doesn't get it, he will scream and yell at the person! He snatches things from people's hands too. When they come to visit, he'll say "gimme that!" Oh hell no kid. Not with me..

We start from infancy.  I always say please, thank you, and welcome to my kids from the very beginning.  Along with other manners too.  In fact, besides mama and dada, my daughter's first word was "thank you." 

My son is 18 months and he has been signing "please" since he was around 13 months.  We are working on "thank you" but he doesn't understand that yet.  

It should start as early as possible.  When my son screams for something I calmly look at him and tell him that mommy doesn't understand screaming and then I ask him if he would like whatever it was he was pointing at and say "Mommy can I have (item)."

From day 1, by example.

We started as soon as possible. We always make sure to say please and thank you and bless you and excuse me and all of that. We also did the signs for please, thank you, and you're welcome (ty and yw are the same sign). Eventually, around 12m DD picked up on how to sign thank you, even though we always had to prompt her. at 15m she learned the sign for please. Now at 19m, she says thank you on her own, but we're still prompting for the please. IMO, manners are learned habits like anything else and the sooner you start, the sooner they pick up on it.

You should start VERY early!! Day 1!

Start early so it becomes like saying i love u mommy. U want it to be a habit not a chore.

When they can actually can talk at the dinner table for starters.

NOW, as you should have been since they were born.  And I don't mean" teach" but "model".  Children are maleable at this age, so the best you can do for them is model everything.  Talk about everything, show theme everything.

My 5yo was saying please and thank you at 2 and still does.....because we do!  We didn't ahve to "teach" him.  He just knows when someone hands you something to automatically say "Thank you"