The hardest part is drilling the holes perfectly center. I would recommend a punch.
That, and either both of the upper ones will be a tricky fit, or one of them will be a REALLY tricky fit. There isn't a lot of room for error.
Measure twice, or three or four times, do the center punch thing on both sides, get one nutsert and bolt in, mount the brace, and then use the brace as a drilling guide for the other three. That way at least you get it mounted even if it's not perfect.
in my car, the upper driver side went in ok, and then, oops, the passenger side, a bit not quite right. I had to dent the next layer of steel under the layer you mount it to, just to get the nutsert in. it's never good to be putting a hammer and punch to your body work.