The first thing you need is a bird feeder. It is best to do this exercise in winter when birds are easily attracted to bird feeders.
The actual count of birds at the feeder can be done a number of ways. The usual procedure is to sit quietly near the feeder and count the number of birds that visit during a set period of time. This period depends on how much time you have available but might be as short as 10 or 15 minutes and as long as an hour. It is most interesting if you count the number of individuals of each species that you see (house sparrows, cardinals, blue jays, etc.). There may be some birds that you won't know, but this would be a good time to learn new birds. Any birds that you aren't sure about can be called unidentified species.
Be sure to have a pencil and paper available so you can write down what you see. It is easy to lose track if the feeder is busy.
For practice, watch this live stream from the feeders at Cornell University and see how many species and how many birds you can identify.