Hold the harness by the straight piece of strapping between the two loops. Look at the two loops hanging down from the waist and identify which is the smaller. This smaller loop will slip over the cat's head and does not need to be unbuckled. The larger loop will go around the cat's chest, and needs to be unbuckled. The spot where the loop connects to the connecting strap should rest just above of the cat's shoulders. With the small loop over the cat's head, slide the harness so the "waist" of the figure eight lies on its back between the shoulder blades. Take the ends of the bigger loop and feed them under the cat's chest. Check for twists and straighten them out so there is nothing to dig into the cat. Then fasten the buckle. If the loop is too small to fit around the cat's chest you will need to ease out some extra length by using the sliding adjustment on the harness. A comfortable fit of this harness should be snug but allow you to insert two to three fingers between the harness and the cat. Use the sliding adjusters to lengthen or shorten the loops individually until you are satisfied the cat is both comfortable and secure. You want the cat to get so used to the harness that it basically forgets that it is wearing a harness. This may be possible for some cats, but not necessarily all cats. It may help your cat to accept a new harness if you leave it beside the cat's food bowl before you try putting it on. That way it will come to associate it with good things.

Summary:
Orient yourself with how the figure-eight harness should sit on the cat. Place the small loop over the cat's head. Connect the large loop around the cat's midsection. Check the fit of the harness. Let your cat get used to the harness while indoors.