I used ladders and stacked boxes, for the general idea. After I was satisfied, I got out my tape measurer and sketched out where I wanted them. 2' from back and side walls. Rears are 4' apart, 4' back from MLP. Sides are 10' apart, 4' up from rears(roughly centered) to the MLP.
Our MLP is a love seat w/dual recliners, so the adjustable tweeters are aimed at the seat further away, therefore having the sound cross over the MLP.
I was lucky that all of my measurements fell in-between the rafters.
I marked dead center for all of my measurements and used a nail, string, and pencil to measure out the circumference. I used a chalk line to line up both sets of speakers with each other, and a chalk line for the center line, so three sets of lines, making sure everything is 90* off of the centerline.
I saved all of the cut 'disks', incase I ever need to patch the holes.
After everything was mounted, I used 1/2 of each box to cover the back of the speakers. We have loose fill as insulation, so I piled it around and on top of the boxes.
Measure, measure, measure! The old adage, "measure twice, cut once", is the truth.