leather

I do it one of three ways, and this works for vinyl too. I use gloss paint, with semi gloss over it, flat paint with semi gloss over it or flat paint with oil from my skin on it to impart a slight sheen.
Lee

Very easy!
To simulate leather, I use any FLAT paint in the proper color. It doesn't matter if it's enamel, lacquer, or acrylic. When the paint is completely dry, burnish it slightly with a piece of old tee shirt, flannel, or even a tissue. This will allow you to achieve anything from a soft glow to a high shine, depending on the amount of burnishing. You do not have to over-coat this with anything after you get the effect that you want.

Pete

What about using leather?
No, I am not kidding. Get a piece of leather from a flea market or from an online hobby shop I get mine from a leather whole sale. Those from the online shops a still to thick in most cases.
You will need a scalpel to separate the top leather skin. That is very difficult sometimes. As a result you have a very thin piece of leather which you can use for the seats and dash.
Have fun.