the brass posts are screwed into the case. the board is then mounted on them.
As boards are different but cases are universal to an extent, the way I do it is to first attach my anti-stat strap. I ALLWAYS
wear cotton. Nylon creates static. Static is our main cause of fuck-up's on builds. I know an engineer who still hasn't figured this one out. He blow ten boards on one computer build last year!! and that is totally honest! this guy is wired most of the time so he don't need nylon...LOL
Anyway that is a must before you start. not saying you'll blow any thing but the risk is higher. Holding the board by the edges (do not grip a board - sweat transferal can and will short a small gap between solder points for instance)
offer it up to where it would be when mounted in the case. Note where approx. the screw holes in the board line up. This then will geve you a good idea of where to screw in your mounting posts. Place your MB carefully out of the way, preferably back in it's anti stat bag/box.
Screw in your mounting studs. the silver ones go around the processor the brass ones do the rest. Pick up your MB and carefully fit it in place. It'll sit slightly of the mounts sometimes cause of the little tensionn springs/lifts on the ports plate (MB plate) some cases come with two types of port plates make sure you fit the right one for your board. they simply pop out and in.
ok also in that bag of bits there would have been some little card rings? you fit one of those to each screw and gently but firmly screw the screw home.
Make sure you tuck the wires that are floating about, power cable, usb cables, on/off, reset, on-board speaker, power pack out of the way...
Next stage is to fit your processor FOLLOW INSTRUCTIONS!!
then Ram ..then have fun with the cables....
watch your usb connections. do you have front case usb? if so the socket you connect them to will run:
in this order
rear of case
front of case
9 is generally a blank
remember those anti stat proceedures though