Remove everything from the garage. Look at the space to determine how you plan to layout your hobby room. You can use painters tape on the floor and walls to give you an idea of what will go where, including false walls, furniture and cabinetry.
Power wash the floor from the inside out to remove any dirt, grime and grease.
Erect false walls, if desired, to create privacy or storage in the garage. Create the walls with 2x4s covered with drywall to create a smooth wall. Attach your wall frames with cement brackets or bolts.
Seal your floor with an epoxy sealer. This will make the garage more inviting as an interior space, and it is durable for the toughest of hobbies. Use safety goggles, rubber boots and a respirator when working with epoxy. Work your way from the inside to the outside. Don't let anyone walk on the surface until it is cured.
Install drywall on unfinished walls. Paint to give the garage a finished interior look.
Install cabinets and pegboards along walls to create storage and easy tool access where necessary. Each hobbyist requires different things, so be sure to evaluate your personal needs to create the perfect space.
Move in. Bring in all furniture, supplies and tools you need for your hobby.