Basic steps :
1. Start Xephyr on a new display, say :1.0
2. export display = :1.0. (You can test it is working by entering xeyes and it should appear in Xephyr).
3. SSH into your server. Whey you use the -X, the display in the ssh session is changed to :10.0 (this is normal).
4. Forward X applications. Fluxbox is started with startfluxbox.