Headsets are the bike components that provide an interface between the moving forks that steer, and the head tube of the frame. There are a lot of special tools for the installation. If you don’t want to spend a lot of money, there are cheaper alternatives (a wooden block and a hammer, for example). Worst case scenario, you can always ask your local mechanic or read some bike books for tool cheats.
Park Tools makes these:
To install a sealed bearing headset first place the bearings, both top and bottom, on the opening where the fork is supposed to enter. There are integrated bearings in some frames as well, so you don’t have to worry about them falling out from the bottom. Put on the spacers, the headset and the dust cap and screw it all down.
To install a threaded headset, the process is very similar to a sealed bearing. The threaded headsets have a threaded bearing race and a locknut that fit on the threaded steering column.
After the head set and fork have been prepared, measure your fork, head tube, and headset to make sure all are compatible with each other. Next, it is important to note how the chosen headset press interfaces with the cup.
Now, apply a thin layer of waterproof grease to the insertion sleeve of the lower headset cup. Next, install the headset cups to the head tube, beginning with the lower cup. Align the lower cup, headset press, head tube, and headset installation tool. Once in line, slowly press the lower cup into the head tube until you feel a firm fit. Do not use excessive torque as it may damage the fork or the frame. Check to see that the cup is fully seated in the head tube. Keep the installation tool in place on the bottom cup. Once this is complete, you may now proceed to the installation of the upper cup.