Dust the leather desk with a duster to remove any loose dirt, dust or other debris.
Rub off dirt stains with an art-gum eraser. Be gentle when using an eraser on leather since too much force may cause surface damage.
Dip a clean rag in a solution of castile soap. A drop or two of the soap in a cup of water is enough to make an effective solution. Wring out the rag so it's only slightly damp.
Rub the leather with the dampened rag. Use a circular motion over the entire surface until all dirt is removed. Buff the leather dry with a clean, dry rag.
Apply a leather finish to the leather once it's cleaned and dried. Leather finish application may differ depending on the product, but it's usually effective to put a dab of the finish on your finger and buff it into the leather. Always read the application directions for the chosen product to ensure best results.