After gaining access to the main hall of the house via Jack's defeat in the garage, you'll need to find a specific item to access the first room that offers psychostimulants.
Go upstairs in the main hall and go through the door on the left side. Follow the hallway left until you come to a door at the very end that turns out to be a bathroom. Amongst other things, including an antique coin, there is a special item in the tub. Drain the water and pick up the wooden shape that was bathing, then open the door to be ambushed by Jack again. Walk around the tub and rush back to the main hall where the first shadow plinth is next to the projector. If Jack is in the same room as you, you won't be able to use the plinth, so make sure he's not actively chasing you. As long as he doesn't see you enter the room, you can hide behind the plinth and repeatedly press
until it allows you to interact with it. Choose the wooden shape and maneuver it into an eagle that matches the painting behind it. The wall will open up to reveal a hidden path. Head down that path to come to a hidden room that contains the psychostimulants.
Once in the room, look on the table in the center to find your first psychostimulants, then open your inventory and choose them, then choose the first icon that is selected to use it. The achievement will pop for you right away.
The psychostimulants highlight every collectable item in a radius around you, even through walls and floors and ceilings. It lasts for a limited time and can be extremely useful if you're low on things like ammunition or healing items. You can also create psychostimulants if you have a chem fluid and some supplements in your inventory. There will be plenty of collectable psychostimulants throughout the game, enough to clear every section of the game and have a few left over. Enjoy your hidden items and your achievement!
-AchieveDad