Most of the stuff only fits in one place.
And there's not much you can do wrong, really.
Just don't forget to put the spacers underneath the motherbord (I once did that...).
Most of the time the toughest bit is to connect the front LED's and buttons to the mainboard. Sometimes this is one plug, but often it's a lot of little plugs. It's all marked, but is't tiiiiiny
.
But, again, if it's just for SCII, all you'll probably have to do is get yourself a nice videocard
