Create your building plan. Find a flat location on your land large enough for the CONEX and have it delivered. Dig necessary plumbing trenches and lay pipe and electrical wire prior to delivery.
Research space-saving appliances and furniture. With about 320 square feet inside, design a simple one- or two-bedroom layout. Decide on the number of doors and windows and where the bathroom will be located.
Measure and cut the holes for the doors and windows, then install them. Make sure one window opening is large enough for an air conditioner if you live in a warm climate.
Cut holes for plumbing pipe and electrical wires. Make the kitchen area back up to a bathroom wall to keep all the plumbing in one location. Your list of electrical needs includes a water heater, air conditioner or heat pump, interior lighting, exterior lighting and a stove. Include some electrical outlets for such items as televisions and hair dryers.
Lay a simple floor of linoleum or carpet.
Insulate the walls if necessary and cover the insulation with drywall or paneling. Paint as needed.
Install your appliances and bathroom fixtures.