The suggested mounting method cannot  yield a level shelf. I tried the suggested method first and it was so bad that I now have 5 extra holes in my wall. I made a mounting template out of cardboard after using carpentry clamps to clamp everything perfectly level.  I got them mounted after much frustration, sweat and curse words. The mounting brackets will scrape the paint off your walls. The instructions do not tell you what size drill to use. 

Until the manufacturer comes out with a template for mounting I would not buy.