How do you make things instant build in CK2? I tried to look up commands but I couldn't find any. From a long time ago I sort of remember there was one but I couldn't find one.
No such option exists in the standard game. If you want things built in your own holdings, you have to order it yourself. Your vassals should upgrade their own holdings by themselves as their own money is available.
If you are interested in some mods there is one that allows you to setup a build order available here: http://steamcommunity.com/sharedfiles/filedetails/?id=345788750&searchtext=
that's about the best you can do I think.
According to CK2's wikia page:
The console is opened by pressing § + Shift. Alt + 2 1 or ~ may also work ( ~ seems to be the majority console button, located beneath esc ). The cheat console is a transparent overlay above the character's tab.
Although, on my keyboard neither works and I have to do Shift + ` + space
quickbuild = Player's construction are finished immediately (One day).