Hi Osman,
In my option, some of the important points regarding painting realistic leather are (1) to use proper colors to achieve contrast and randomness in color (2) to build up textures gradually
As for colors, depending on the type of leather (brown, red, black and etc.) you want to paint, try to prepare a pallete having required colors with a wide range. For example, for brown leather, you can have colors from very dark brown or black brown, cholocate brown, orange brown, biege brown to very light sand brown or tan yellow, just name a few and the list is not limited to these. Don't be afraid that the contrast is too much at the beginning as you can alway tone it down if needed by using glaze at a later time. You can use technics like wet blending and stippling to quickly achieve some color randomness and contrast presenting what you usually see in authentic leather materials.
Painting textures is also important for leather. Try to draw very thin lines or patterns as textures on the leathers and also usually some worn effect or even cuts on the edges if you are trying to paint some old leather. Just give it a try and you will find that it is not difficult to achieve such an effect as long as you have some patience and a steady hand.
Just my two cents. Hope this helps.
Thanks,
Jason Zhou