OK, expander plugs come in two major design types;

1. The plug and top cap are all in one unit and held together with a long bolt and a spring to preload the expander. These use the adjusting bolt to expand the plug and adjust the headset at the same time. The adjusting bolt head will be flush with the top cap and look just like a regular star-nut adjusting bolt. You install the fork, add spacers and the stem, then stick the expander plug in and tighten the bolt to both grip the steerer and preload the headset. Look and Profile Design make this type. Here is a picture of Profile's:

http://www.profile-design.com/profil.../gap-cap-.html

2. Two piece types. The top cap has a 6 mm hex hole in it and has a cylindrical tube on it's underside that is internally threaded. The body has the actual expander and a externally threaded extension on top with a 5 mm hex recess. You remove the top cap and insert the body into the stem and tighten the 5 mm hex to expand the plug and grip the steerer interior walls. Then install the spacers and stem. Finally, thread the top cap down over the inserted body and tighten the top cap with a 6 mm Allen key to preload the headset. FSA makes this type and here is a picture of one:

http://www.biketoolsetc.com/index.cg...em_id=FS-CPSR8

The depth to insert this is sort of by eye. Insert the expander body far enough that the top cap won't bottom on it but the top cap can reach the threads enough for several turns. My FSA expander is set so the top of the body's projection is 8 to 10 mm below the top of the stem. Here is Salsa's .pdf on cutting a carbon steerer and installing this type of plug.

http://www.salsacycles.com/pdf/SalsaCarbonSteerer.pdf