The answer is easy but the training process is slightly more complicated. There are several sites which appeal to most owners. Another approach, especially for male dogs who like to lift their leg, is to set aside a small square of gravel with a post in the centre. These areas can be quite attractive conversation pieces using a tree stump or an imitation fire hydrant as the central post. A third alternative, which is my method of choice since I rent, is to train the dog to urinate in the soil under a low-lying tree or bush.
Dogs have a habitual behaviour pattern they follow prior to eliminating.
Although it is slightly different for each dog, it usually includes some combination of sniffing and circling. You will have to observe your dog so that you know exactly when your dog will void. While the connection between the phrase and the act can be made once the act is initiated, the fastest and easiest connection is made if the phrase is vocalised just prior to the act. For the quickest results, it is also significant that you try to do this each time your dog goes outside. You will know you are ready to go onto the next step when your dog takes the initiative to urinate when he hears the command.