My son is 2 1/2 and we have completely potty trained him during waking hours in the past 5 days. For months we have talked about where poopy and peepee go. We have encouraged him to use the potty and have given him naked time (when we're home and inside, just letting him run around naked and he'd go in and use the potty on his own when naked).
This Saturday, we went straight to underwear. We were out and about doing our "normal" stuff and just carried changes of clothes and the potty seat (the kind that goes on the regular potty). As soon as we get to a store or someone's house, I seek the bathroom out and take Cody right away. Every 30 mins we ask him if he has to go as a reminder, if he doesn't go then we take him every other time. Saturday while we were out he had 2 accidents, Sunday he only had 1. He's now gone 3 days without any accidents.
We reward the good times (m&ms for pee, hot wheels for poop and at the end of an accident-free day, something else a little more special). When there are accidents we don't punish, but we do let him know it makes us sad and its not good to pee in underwear.
Good luck!