Good old Martha taught us that rather than buying those ridiculously expensive anti-slip mats for a rug, just turn it over and run a few lines of acrylic-latex caulk every 6 inches or so. Let dry and flip over and your rug wont be going anywhere!
Duh, why didn’t I think of that? Hang a picture/object level by putting painter’s tape on the level and marking where the object’s holes are. Put the level against the wall and mark the wall based on the painter’s tape markings.