Start with e2 in the D90 Menu, where you select Commander Mode.
On the SB-600 you need to hold down "ZOOM" and "-" simultaneously for two seconds. Next use the "+" or "-" to scroll to the squiggly line with an arrow at the bottom icon and use either "ZOOM" or "MODE" to turn it on or off. Then hold down "ZOOM" and "-" simultaneously for two more seconds and the SB-600 will now be in Remote Mode.
Finally, align Channels and Groups between the Commander Mode (in this case, on the D90; but it is also available on the SB-800 or SB-900) and the Remote Mode Speedlight. Typically you start with Channel 1 and set the Remote Speedlight to Group A and any additional Speedlights to Group B or C.