A simple potting shed, in its barest element, is nothing more than 4 walls and a roof. In warm climates, you may not even want to close the walls and will leave them open. Keep in mind, though, that blowing wind and rain can be annoying to you and damaging to your plants. This steps listed below are for a potting shed with exterior siding, but leaves the interior walls open.
Step 1: The Design
Draw out a pattern. A good size is 10 ft. long by 8 ft deep. Plan for studs at 24 inch centers. The roof slope can be achieved by making the front wall slightly higher, and you can cover it with sheets of clear or opaque plastic. Don't forget the doorway, and add extra material into your calculations for framing it.
Step 2: Material Cutting
Build the garden potting shed just like a store-bought kit. Pre-cut all of your material and stack it near the location where it will be used. By having everything cut to size ahead of time, you will be able to assemble the walls quickly, without countless stops for new cuts. If you do have to make adjustment cuts during the construction, they will be relatively minor compared to stopping for every stud.
Step 3: Wall Assembly
Mark the stud layout on your top and bottom plates. Keeping the studs on a 24 inch center reduces the amount of lumber used, but still allows the siding to "break' on studs. Nail each wall together on the ground, and then stand it up. Connect the walls at the corners and add horizontal bracing to give them more support.
Step 4: Simple Slope Roof
Lay rafters across the top of the potting shed at 24 inch centers. Cover the roof with plastic or vinyl sheets. These sheets have holes where the fasteners should go to avoid damaging the material. If your material is shorter than the roof, put the short pieces on first. Then, place at the lower edge and overlap whole sheets over those.
Step 5: Potting Shed Siding
Apply T-111 siding to outside of the potting shed. Not only will this give you some protection against the elements, but it will also give the walls a lot more support strength. Even if you are not able to cover all 4 sides, it is best to put siding on the rear and ends to keep the shed stable. Paint the siding to prevent it from weathering, using a white or light color. Dark colors may absorb too much heat in summer months, making the shed almost unbearable to work in.
Step 6: Customizing
Once the basic shed has been built, customize it for your own needs. Add shelves for your materials, and racks for hanging planters. Add an access panel in one wall so you can transfer compost or soil inside without having to go in and out the door. There are many great ideas for customizing a potting shed. You can build a stool and add hooks to keep everything well organized.