When I built my pack, I used a piece of 2" angle aluminum and used a hacksaw to cut it down - it worked really well for me.
Picture 1 shows the brackets after being cut and the edges sanded down.
Picture 2 shows the 4 brackets mounted to the shell.
Picture 3 shows the motherboard mounted together. (the windows are because this was built to house a computer...I can post more pictures of that later, if requested)
As far as actually mounting the brackets; I knew where the holes needed to be in the shell, so I started by making those holes. Then I held the bracket in place and marked where the hole lined up on the bracket, and drilled that hole next. Then I used the bolt and nut to hold the bracket in place, laid the motherboard on top, and drilled my mounting holes there. Because of the nature of my build (having a PC in it) I wanted the motherboard removable so I did not rivet mine in like most builders on the site; so to keep the nuts from spinning and allow me to tighten up the bolts, I glued the nuts in place on the brackets and then bolted it together.