Leather dries out when it gets wet and then dries improperly. Improperly dried leather gets small cracks, and if left untreated, the small cracks expand and ruin the entire piece. Applying a high-quality leather conditioner will restore the softness to dry leather, and will shrink small cracks, making them less noticeable. Large cracks will still show, but conditioned leather will be softer and will last longer than untreated leather because the cracks will not spread as quickly.
Wipe any debris or dust off of the surface of the leather with a soft cloth.
Wet a corner of the cloth with the leather conditioner.
Rub the conditioner on the dry leather, working it into the cracks and thoroughly coating the surface of the leather. When the cloth starts to feel dry, wet it again. If the leather is extremely dry and cracked, it will seem like you are using a lot of conditioner, but it is normal for dry leather to absorb more than undamaged leather.
Buff the surface of the leather with another soft cloth to remove excess conditioner and to polish the leather.
Different kinds of conditioner are available for different leather uses. Mink oil works good for things like work boots but is too greasy for leather furniture or a jacket. A good rule is to buy your conditioner in a shop that sells similar items. Go to a furniture shop or upholsterer to get conditioner for leather furniture, a shoe shop for footwear conditioner, and a shop that sells leather clothing to get leather clothing conditioner.