Originally Posted by Lacelady

I made one a few years back. I measured the mantle shelf, and divided the long side to decide how many triangles to make, and put one on each end to wrap round the corner, and then one on each short side. I made the triangles double sided so that the whole thing is reversible, and sandwiched the raw edges of the triangles between two layers of fabric cut to the shelf top, with seam allowances added.

Hope that makes sense. (I trimmed with bells for some fun)