Start out by printing out the design you want to transfer to the leather. Moisten the leather using a sponge and let it sit for a few minutes until the surface is soft. You can place the printout on the leather and trace it using a modeler. If you decide to lift the paper to check if it transfers correctly, make sure the whole image is aligned properly when you put it back down!
Note: depending on the type of paper you use, it might tear when you trace it because of the moisture in the leather – so be careful not to apply too much pressure.