If your headset was loose and you tightened it, proceed to step 4b.
4b) Test the steering by squeezing the front brakes.
5b) Rotate the handlebar 90 deg.
6b) While still squeezing the front brakes, gently rock the bike forwards and back. Try to "feel" any looseness in the headset. If the headset still feels loose, repeat steps 2-6b.
7b) If you're happy with the adjustments, you'll want to tighten the stem bolt to 13N*m if you have a torque wrench. If you do not, then you'll want to tighten it "pretty darn tight".