How To Slave an SB-800
1. First, turn on your flash.
2. Now get into the submenu by pressing and holding the "sel" button for 2 seconds.
3. Arrow right (the "single tree" button) to get the upper right quadrant highlighted.
4. Push the select button again.
5. Scoll down ("-" button) until "SU-4" is highlighted.
6. Push "sel" again to select.
7. Push and hold "sel" for 2 seconds to get out of the submenu.
8. Your flash will now say "REMOTE."


You are now in SU-4 mode, a legacy, wireless, quasi-TTL mode for which the SB-800 is backward compatible. But you are not going to use it for TTL. By pressing the "mode" button you can toggle between automatic and manual. Choose manual.
By pressing the "+" or "-" buttons, you can change power in 1/3-stop increments all the way to 1/128 power.
You now have a flash that'll slave to any other flash. You want to use both in the manual mode, so they will not influence each other's output.