Prepare the exterior of the garden shed. Resurface the exterior walls with cedar shakes, planks of old barn wood or aged brick. Attach a cupola on the roof for a Victorian era-themed outhouse. (See Reference 1.) Consider adding wire-covered ceiling vents for fresh air in the garden shed.
Decorate the garden shed door. Cut a crescent moon in the door for classic outhouse character. Attach a piece of Plexiglas inside the cut-out to keep the shed weather-resistant. Hang a vintage sign on the exterior of the door to welcome guests. Choose old lavatory signs from an antique shop.
Decorate the interior walls of the garden shed. Use old newspapers and catalog pages as wallpaper. Be sure to leave an old mail-order catalog in a visible spot, with a few pages already torn out from previous outhouse users.
Add a bench seat to one wall of the garden shed. Allow the top of the bench to open for storage of gardening tools. Decorate the top of the bench seat with an old toilet seat lid. Hang funny bathroom sayings or potty-joke cartoons near the bench seat.
Add accessories to the shed to give it a bathroom feel. Frame old advertisements for toilets, plumbing and toilet paper and hang them on the interior walls. (See Reference 1.) Hang an historic lantern from the ceiling for a warm glow in the shed. Make curtains out of burlap feed sacks to add privacy. Hem a simple pocket along one edge and use a wooden dowel rod to hang the old-fashioned window coverings.