The Test Sign is a tool used in Portal 2 to hint to the player what elements will be presented in the level, and the progress they have made out of the total amount of levels.
To create a test sign, first place a func_instance on the wall where you wish the sign to be. Then choose the VMF filename "instances/signs/info_sign_sp_clean.vmf". Name it "level_sign"
Then place a trigger-texture before the player will run up to the sign and tie it to a trigger_once. Make the output for the trigger_once "On Trigger | level_sign | instance:info_panel_activate_rl;Trigger | none"
Hammer may indicate that this is invalid, ignore it.
As of now we do not have information on how to put information on the sign, this will simply put up a blank sign with "00" as the level number and no description. Please add any information you might have!