Most children have a pattern as to when they go to the bathroom. Some go after they play, most go when they wake up, or other intervals such as those. In order to establish the pattern of using the toilet, you must first have successful experience. So whenever the interval at which the child usually goes to the bathroom approaches, take the child and sit them on the toilet. Have a powerful reinforcement on hand such as chocolate, fruit, or a very nice toy.
Food is the best reinforcement because it is immediate gratification.
Eventually the child will let out some urine or defecate, when this happens make a big deal out of it. Give the child tons of reinforcement, tell them how good they are, then take them out to play. After the child will repetitively go when you take them to the toilet, start putting them right next to the toilet, allow them to pull down their diaper and sit on the toilet. After this, have the child follow you to the toilet and go through the process on their own. It is important to repetitively reinforce this behavior.