i need a tutorial on how to decorate the surface of leather? Answered

With leather, you can burn it with a leather burner, or a wood burner.

nip by your local http://www.tandyleatherfactory.com/ and ask about classes, or read any book by Al Stohlman

I can think of two methods to make patterns in leather:

Pyrography: Using a flame or a heated tool to burn a design. Note, this will smell horrible.

Embossing: The simple water method below works for simple shapes with large features. More detailed designs require a metal stamp or die and mechanical pressure. For one small run, you could make a stamp out of hardwood or some other strong medium, set it in place and clamp the piece onto a table or something.

Link:
https://www.instructables.com/id/Leather_iPod_Touch_Case_using_Water/