Start by taking a sip, and then move the beer around in your mouth so that it coats your tongue, teeth, gums and the roof of your mouth. Since different taste buds are located in different parts of your mouth, it's important that the beer comes into contact with every part.
Swallow, and then consider the aftertaste. What flavors remain in your mouth even once the beer is gone? Often the aftertaste of a beer is very different to the tastes that you can identify when you're drinking it.