Insert the expansion plug into the steerer; make sure that it requires a little effort to insert, then tighten it to the fork manufacturer's recommendation. Put an appropriate amount of spacers under the stem, then install the stem, and put a 5 mm spacer on top of the stem (this is my personal preference). Then install the top cap / bolt, and tighten the headset with the stem steerer bolts loose until you take all the play out. Once everything is nice and snug (but not too snug), tighten the stem steerer bolts, then give the top cap bolt just a nudge to firm it up.

There are many online videos that show this process, too.

P.S. Looks like I type slow, whole bunch of folks beat me to it...