Leather is the product of tanning animal hide. Animal hide is just another way of saying animal skin, so leather is tanned or dried animal skin. Transferring ink to leather work is just like transferring ink to skin the way that a tattoo artists does when putting the pattern on the skin before getting started. Likewise, the easiest way to transfer ink to leather will be the same way that tattoo artists do it, using tracing paper, ink, and ultrasound gel or an unscented gel deodorant.
Draw or trace the drawing that you are looking to transfer onto tracing paper using a regular ballpoint pen. Tracing paper is a thin, see-through paper available at any hobby supply shop or online.
Dampen the leather with water. If you were a tattoo artist prepping skin, you would use rubbing alcohol instead of water, but the leather doesn't need to be prepped with rubbing alcohol. Do not soak the leather in water, just dampen it.
Apply ultrasound gel to the leather and then rub the gel in around the entire area where the ink will be transferred. Instead of using ultrasound gel, some tattoo artists use unscented gel deodorant because it is less expensive and easier to find. If you are using the deodorant instead of the ultrasound gel, then simply rub the deodorant onto the are where the ink will be transferred.
Place the tracing paper onto the leather with the ink-side facing down and then press the tracing paper onto the leather with your fingers. Do not move the paper around as this will smear the ink. Instead press the paper straight down.
Lift the paper up from the leather carefully to inspect how the ink is transferring, if the ink has not transferred completely, then press the tracing paper back onto the leather and repeat the previous step. If the ink is transferred to the leather, then remove the tracing paper completely.