Measure the length and width of your garage. If the walls are unfinished, you will need to insulate and finish them, so subtract an inch from each measurement to allow for the drywall thickness.
Draw out the shape of your garage on graph paper with 1/4-inch squares, using one square to represent each foot of garage floor space.
Outline the following shapes, using the same scale, on a separate sheet of graph paper, and shade them in: 30 square feet (five by six squares) for a treadmill or elliptical trainer, 35 square feet (five by seven squares) for a single-unit gym machine or free weights, 10 square feet (two-and-a-half by four squares) for an exercise bicycle, 20 square feet (four by five squares) for rowing machines and stair climbers, and 25 square feet (five by five squares) for ski machines. Choose only the shapes you intend to use in your garage.
Add another shaded six-by-six square area for stretching, or combine this with your free weight area by extending it to six by eight squares instead of five by seven.
Outline and shade in a shape representing any storage cabinets you plan to add for items such as towels, bottled water, yoga mats or gym shoes. A one-and-a-half by three-foot cabinet (represented by the same number of squares) should be more than adequate for most garage gyms.
Cut out your shaded shapes with scissors and arrange them inside your garage outline, leaving at least two squares of free space between each shape.
Arrange the shapes in several different ways until you are happy with the layout. Consider how you will move from one station to the next, and consider placing items like stationary bikes or treadmills where you can look out a window as you exercise.
Tape the outline of each shape on your garage floor, calculating the measurements from your graph paper into accurate sizing in the real garage space. Once the areas are outlined with masking tape, walk between the spaces to see if it feels comfortable or too crowded. Adjust your design accordingly, first on paper, and then on the physical floor.
Plan to insulate and heat the room if it is not already insulated and heated so you can work out comfortably all year round. Air conditioning is also important if you live in an area with extreme heat.
Choose flooring that gives some cushioning for your stretches and aerobic exercise, such as thick mats. This will also help insulate you from the cold concrete and provide a removable surface in case you want to convert the room back into a garage.
Design adequate lighting and electrical circuits for powered machines into your home gym. This may require an electrical contractor to complete properly because many garages do not have more than a single electrical circuit.
Cover your garage ceiling with drywall rather than keeping the rafters open and insulate the area above to maintain the optimal temperature while exercising.
Place a large mirror along one wall or in front of the garage doors. This helps hide the doors and will allow you to see your form as you work out.