Stripping off the old wall covering is usually wiser than leaving it on. New coverings adhere better to stripped-down surfaces. Depending on the wall covering and the kind of wall it's on, there are several ways to approach the job.
But you can successfully paper over old wall coverings, although it's not always a good idea, because the moisture in adhesives can cause both the old and new coverings to peel away from the wall. Also, if previous strips of wall covering have been lapped at the seams, these lap marks will show through the new covering. If you still want to paper over old coverings, as necessary, sand the seams smooth, tear away any loose strips, and re-paste loose edges around butt seams or defects before applying the new covering. If you're papering over foil or vinyl wall coverings, go over the shiny areas lightly with coarse sandpaper and then vacuum or wipe the sanding dust off the wall.
Now, if you've decided to strip off the old stuff, here are detailed instructions.