Measure the length of the railing. In order to create a solid surface to cover, plywood will need to be used to create a backing. You will need to cover both sides of the railing with plywood for freestanding railings or just one side if the railing is attached to a wall. Also measure the length and width of the top part of the railing to create a cover for it.
Purchase the plywood in the sizes needed. If you have a very long railing, consider purchasing several smaller sheets to make it easier to install. You can have the lumberyard or hardware store cut the plywood to your desired size to make the process easier.
Attach the plywood to the sides of the railing. Using a drill and wood screws, attach the plywood sheet to the railing. You can run the screws through the existing banisters if they are wooden. For metal railings, attach the plywood sheets to the wall instead or to each other if the railing is free standing.
Install the top piece of plywood over the top of the railing. Again, using your drill and wood screws, attach the strip of plywood that will cover the top of the railing. You should now have a completely covered railing that is ready to be finished.
Staple or glue the fabric or the wallpaper to the plywood. It is best to use wood glue instead of the sticky backing of the wallpaper to ensure that the edges will not roll up. If using fabric, pull the fabric tight and staple it along the bottom of the plywood to hide the staples.
Paint the plywood if desired. Alternatively, you can paint the plywood instead of covering it with fabric or wallpaper.