This can be done easily and quickly at Suzuka (Short), where there is only a single DRS zone that begins just before the finish line onto the pit straight.
DRS is activated by pressing
at the white DRS sign on the side of the track: you'll also have a distance countdown in your HUD and the mini-map will show a highlighted area of where the DRS zone begins to help you time it perfectly.
I unlocked this in an online one-shot qualifying session.