I’m describing this method as a way to fix a mistake. 
This method only works on small pieces such as the wrong sized stair riser I hooked recently. I whipped it with my usual finishing technique instead of folding the edges back and whipping.
Since I had already sewn on the Velcro strips (a difficult task that I was not going to undo) I added the hook side that has a sticky back. Normally this is the point where I would remove the protective backing and stick the stair riser to my stairs. But alas, this one is too large.
Years ago I found some 3M hooks on sale. I think they were for Christmas lights. The hooks were too small to do anything with, but the sticky parts have been priceless. I added a bunch of them all around the edge. Then I let it sit for a few minutes. The glue gets stronger with age. I think the packaging recommends 30 seconds or something like that.
Next I test fit the stair riser into place. I noted how much room there was on each side and how far below the mirror I wanted it to be. I removed the second side of protective paper from the 3M strips and gently stuck it to the wall. I then stepped back to be sure it still looked level, made a few adjustments, rechecked and then firmly pressed on the tape for several seconds in each place. My goal was thirty seconds, but geez that took too long. I probably got a good 10 seconds of pressure on each 3M piece.
I rechecked it in about an hour and since it was still up, so I hoped for the best. Twenty four hours later it is still up.
If you are thinking that this is a crazy way to get there, you are right. Remember that I started with a stair riser complete with the Velcro already sewn on but it did not fit on my stairs. With this method, I can change my mind without damage to the wall. I can always use the sticky Velcro later to attach it as a stair riser if I discover one of the stairs is taller than the others. I like options.
