The headset is the set of components on a bicycle that provides a rotatable interface between the bicycle fork and the head tube of the bicycle frame. The tube through which the steerer of the fork passes is called the head tube. A typical headset consists of two cups that are pressed into the top and bottom of the headtube. Inside the two cups are bearings which provide a low friction contact between the bearing cup and the steerer. (wikipedia)
I had to do it, the disassembly was almost easy, I lost one of the bearings but that problem was solved with a trip to the next door bike shop. When I tried to reverse the process I spent half an hour googling info about this subject until I came across this picture that saved the day. I couldn't open the link www.evanscycles.com but the picture still exists somewhere. You can find the same image on this guide.