First make sure that the fork length is cut correctly. The top cap should not bottom out on the fork when tightened. Next assemble everything loosely. Snug down the cap such that the fork turns freely but with no free play. It should not take much force on the top cap bolt. Now tighten the stem bolts, tight but crushing tight. Alternate tightening the bolts a couple of times if your stem has to until they fell like they are even in tightness.

The top cap takes the play out of the headset and the stem holds everything together.

Styrrell