4 Using a sharp hacksaw blade, cut the steerer 3-5mm below the line marked by the top of the stem to account for the space needed for proper bearing adjustment. Before making the cut it is worth holding the steerer up to the frame one last time to make sure the length still makes sense. Cut it straight and clean any burrs with a sanding block or flat file. A pipe-cutter can be used and ensures a straight cut but may require more post cut clean-up.
5 Installing the starnut of a threadless headset requires nothing more than a hammer and a straight hit, though there are tools and guides to make the process easy and the alignment straight each and every time. The trick without a guide is getting the first row of “stars” to bite into the inside of the steerer at the same time on one shot, and then slowly tapping the starnut in straight. If the first row of teeth grabs at the same time but the starnut is not perfectly straight, I’ve found that you can straighten it using a hammer and punch to push on one side or the other before engaging the second set of teeth and sealing your fate. You’ll want the starnut to sit about 15mm below the top of the steerer.
6 Reassemble the fork/frame/headset with your chosen spacers and stem, this time finishing with the top cap threaded into the starnut. Adjust the bearing via the tension on the top cap before checking stem to fork alignment and tightening the stem clamp bolts.
There is a wide variety of headsets available beyond the design shown here. Besides the common 1” threaded and 1 1/8” threadless designs (shown), there are other steerer sizes and threading combinations out there, along with newer models with integrated bearing cups and even newer tapered steerer tube designs. When in doubt, consult a shop mechanic.