Well I can't tell you how to do it step by step entirely, but here's a good start. Examine the edges and the side that faces in for screws, or small plastic circles or squares. The small plastic circles and squares are spots that have screws beneath them. Look on these spots for a small hole or something to pop the plastic out to get to the screw.
If you post the make and model, I might be able to give you more information.