How do you put fork oil in a fork?

Typically there is a plug in the top of the fork that can be accessed by removing a metal or rubber cover, but it really depends on the model motorcycle you are working on . . . the best resource is a clymers repair manual for most models. These can be purchased online (usually on ebay) for $10 - $20. If you plan on working on your own bike, these are a must have!