Alright. I've got my map here, and I want to make it so that after 20 minutes a door opens.
I keep hearing that I should use the trigger_random. And I skulked around the forums to see if this has been answered. But none of the issues related solved my question.
Could someone give a GOOD description of how to do this?

What you could do is make a trigger_auto with the delay before trigger set to 1200 (60*20=1200).

trigger_auto would be simpler, but you could use trigger_random too. set short and long fire time to 1200 and 1201 then flags Start On and Remove from File. just use trigger_auto though, a lot easier.

if you google both of them you can find a good description about them.

If u ever need a good description of what an entity does, use Valve Editing Resource Centre it has a comprehensive list of all half life entities and there flags. It also includes specific mod entities for some mods, unfortunatly tho, not one for NS.