Summarize this article:

Before you can scratch your dog's stomach you will need to wait for it to show its stomach to you.  While you could try to make it roll over, forcing the belly rub is not a good idea.  Play with your dog and calmly wait for it to show you its stomach for scratching.  Always wait for your dog to show its stomach. You dog will naturally roll over and present its stomach if it wants a belly rub. A dog who wants a belly rub will appear loose and relaxed in regards to its body language. After your dog has presented its stomach and you feel that it is comfortable and relaxed, you can begin to rub its belly.  As you rub the dog's stomach keep an eye on its behavior, looking for signs of enjoyment or discomfort.  It's normal for some areas of the stomach to cause your dog to kick when you scratch them.  Always scratch or rub gently as the stomach is a sensitive area on a dog. Feel free to rub your dog's stomach however you'd like.  Try rubbing in circles, short scratches, or longer motions up and down the whole length of your dog's stomach. After you have scratched your dog's stomach for a while you may decide it's time to stop.  Your dog may try to get more belly rubs out of you and it's fine to continue if you want.  However, if your dog tries to get up or move away from you it is time to stop.  If you stop rubbing and your dog moves away from you, it's time to stop scratching its stomach. Feel free to stop scratching your dog's stomach any time you wish. Although dogs love a good belly rub, they also enjoy receiving attention on other areas of their bodies.  Try petting, patting, rubbing, or scratching some other places on your dog to see what they might enjoy.  As always, work slowly, touch gently, and make sure your dog is truly enjoying it.  Your dog might enjoy a rub or scratch under or around its ears. Most dogs will like a good neck rub or scratch. A dog might enjoy it when you scratch its chest. Try scratching under your dog's chin. A dog might like it when you scratch the area of its back just above where its tail starts.

Summary:
Wait for your dog to present its stomach. Rub or scratch the dog's stomach. Stop when you are done or your dog becomes uncomfortable. Scratch or rub other areas on your dog.