As leather seats and furniture get older, telltale signs of that ageing begin to show up on the surface. This damage, which generally takes the form of fissures or cracks, is mostly reversible, depending on the leather's age and amount of wear and tear.
Take a shoe brush or dry cloth and brush away any dirt and loose particles on the piece of leather. Clean the leather with a mixture of mild soap and warm water. Be sure to dig into the nooks and crannies so that the full surface has been evenly cleaned. Wipe the leather dry using a clean cloth.
Use denatured alcohol and a clean cloth to wipe the leather down. This removes any soap residue still left on the leather, but will also remove any of the original finish as well as some of the leather's colour. Afterward, use another clean, dry cloth to remove any dirt, as alcohol might still remain on the leather.
Apply the filling compound from the leather repair kit into the cracks with a teaspoon or similar blunt object that won't rip the leather. Rub the compound deep into the cracks to fill them, then let it dry, which should take from 5 to 10 minutes.
Sand the repair compound lightly into the cracks with a fine grade of sandpaper. Work the compound into the leather cracks gently; be careful not to press too hard or else you could cause more damage to the leather. Wear gloves while doing this, so as not to damage your hands. Disposable rubber gloves, like those commonly used to clean kitchens and bathrooms, are perfect for this step.
Use a soft rag or sponge to apply the repair solution from the leather repair kit to the entirety of the piece of leather you're fixing. Use light pressure to work the solution into the leather, so as not to damage it. Let the solution dry, then inspect the leather's surface. If you still see cracks, apply more of the filling compound to them, then apply more of the repair solution and reinspect.
The item will be more delicate than a new item. Wear vintage shoes, coats, belts, on special occasions. A few cracks add character to a vintage item. As long as the item does not look ramshackle, then a few cracks won't hurt the look at all (e.g., on an old pair of "Granny" boots, a WWI-era sword belt).
Do not use your spoon if the leather has a grain or pattern.