1 Loosely assemble the headset cups and press, being mindful in your selection of tool inserts to pick one that does not press directly on the bearing surface, or on the bearings themselves of integrated cartridge models. With the cups aligned properly and sitting straight, slowly start pressing the cups into the frame while making sure they enter the frame straight. If alignment is a problem it may help to push the cups in one at a time. It will take some effort to push the cups into the frame—if they slide in too easily or feel like they don’t want to fit at all it is best to consult a shop for advice. Make sure the cups are seated absolutely flush with the frame.
2 Install the crown bearing race on the fork. Another tight fit, there are tools available to make this process easy but many find that a section of pipe, an old stem or a hammer and punch can do the job. Whatever the tool of choice, be careful not to mar the surface the bearing rests on. Press the crown race flush.
3 With a new fork you’ll most likely need to trim the steerer tube. Assemble the frame/fork/headset and add the spacers and stem you plan on using on the bike. I always err on the side of too much steerer tube on new builds, trimming them and removing extra spacers later if needed. The old adage “Measure twice, cut once” applies here, as you can’t make a too-short steerer any longer. Mark the steerer at the top of the stem.