please help – setup external monitor on a laptop with no monitor

Sony Vaio monitor is liquidated – kaput, finished. Bought an external monitor which is working cables working – but the vaio refuses to see it, no recognition. It had been set up by using a 3rd monitor – but I wasn’t following how it was done. Do I need another monitor to get this going or can I do this in the dark somehow? When is the optimal time to plug the new monitor in (before or after rebooting?) when to try Fn key? Any ideas?