To be honest, if I'd like to do that properly, I should do the whole article about types of leather, how do they age and how do they look when old and worn out. But that's blog about painting miniatures, not leatherworking, and because I don't wanna bore you to death with this theoretical part, I'll simply reffer you to the the article about leather on wikipedia and show few pictures of new and used leather I found in internet.
That's not all leather types of course, just the most common these days. So, I'd say, you should do some proper research by yourselves, especially if you're trying to paint some less common types of leather, i.e. deer leather used by native americans, kangaroo leather or something more exotic and luxurious like snake or crocodile leather, or perhaps fish leather (it does exist, seriously, and looks pretty cool).
When I paint leather, usually I start with fairly light colour (beige, light brown, ochre, yellowish shades of brown), and then slowly build darker parts with washes (devlan mud, black, sepia, ogryn flesh, and even green and blue if I'm going for colder shades.
I don't paint too neatly, rather use irregular spots and uneven layers mixing various colours.
To add more worn look I go over the glazes with the base colour or some darker shades of brown. When I'm more or less happy with the lights and shadows I add some highlights (washing them lightly in the process to make them blend better).
Lots of washes and paint layers applied unevenly to the surface, helps me to build three dimensional texture of the leather. But if you want even more texture, to mimic really rough leather, you can use almost dry brush with a bit of colour and dab it in the surface where you want that particular colour and roughness. This technique is very impressive especially on bigger scale models, where it looks very natural.
Adding damage to the leather elements
Normally leather while used gets darker (due to contact with oil, water and other stuff) so if we disturb that top layer, we can often see the lighter colour showing beneath. To recreate that effect we can make sure that the whole surface is showing some signs of being used.
This type of damage is achieved mostly by uneven application of paint in the process of adding shadows. But if that is not enough you can add some more using almost dry brush with small amount of bright beige and dab the leather to create random spots. Just concentrate on the places which are most likely exposed to that kind of damage.
To create damaged and worn out edges, I use bright beige (off white if the overall colour is rather light) and simply paint thin lines in the places where I think leather is most likely going to be damaged. And then glaze them a bit with one of the washes to get more unified look. I use that method mostly on belts and straps or on the edges of bigger surfaces if that looks believable. Thin lines of damage can be added in the middle of leather elements, but we should be rather reasonable, and do that only if that type of damage could be explained by the usage of the item (i.e. leather armour). Just remember that the whitish lines are not holes (crackles) in the leather but represent damaged top layer of the surface.
As you can see the technique is really simple and requires only some practise and, what's very important, proper research of the type of leather you want to recreate.
I don't really have any step by step pictures of how I paint leather, but I gathered some, to show you more or less what I'm doing.
Menhom Dark Shadow (finished piece)
Pegaso boys (WIP)
Mr Grinder (WIP)
Contemporary leather can be dyed in any colour you can imagine. So when you paint modern or s-f miniatures, feel free to choose whatever suits your colour sheme.
With historical pieces I'd rather go with natural shades of brown, gray and black, as I have no idea which colours were avaliable back in the past. If you are really curious about that, feel free to do some deep research on the subject:) I'll just go with natural browns. It's safer this way, and I won't have to explain to people that according to some old and dusty books, this shade of colour was avaliable on leather in that particular times;]
And I guess that's all I got to say about painting leather elements. I hope you'll find some tips and inspiration in this tutorial.