Small Steps. First teach them to recognize when they need to go (naming it when you see it, as specific as you can). This communication is key. If stalled, try to get them to go into the bathroom, even if still in diapers. Then sitting on the potty. Model toileting all along the way.
Steps to. Independence is a book that helps you implement the good advice given already. Can your child walk from room to room, say/sign " pee-pee, poo-poo & potty" & pull his pants up & down? Have a potty party for ~ 10 minutes, 15 minutes after a daily meal. A little looster provides support, preferably with knees up a little higher than 90 degrees. Blowing bubbles is good; casual is key.