M.O.
18 months is really young. Some toddlers (almost all of them girls) do train this young, but it's uncommon. If you really think she's ready in every way, though, here's what I'd recommend:
Take her with you to buy a potty, and let her pick it out. If she's not excited about any of the potties in the store, get a plain one and let her decorate it with stickers. Go on and on and on about how beautiful and wonderful her new potty is.
Then, whenever she's in the house, let her just run around with no pants on. Be prepared to clean up one or two messes, but if she's truly ready, she'll find her way to the potty pretty quick.
Once she's got that down, she graduates up to wearing underwear (just underwear, not with pants yet). Let her choose some "beautiful big-girl underwear" and make a big deal out of how wonderful it is.
Once she can make it to the potty with underwear, she moves up to underwear with pants, and then she's trained.
Other things to keep in mind are to make the potty a fun place to sit (read to her when she's there), never, ever shame her or make her feel bad for accidents, and really celebrate "successes" -- lots of parents to the "happy potty dance."
I also want to say -- I swear by this method, but I swear by it for 2 1/2 year olds. The risk with an 18-month-old is that it won't work, and that will make the next attempt much harder. Kids who start too young often learn later -- and with more trouble -- than kids who start a little older.