Just a few drops in whatever acrylic paint you wanna use I use don't use alot of fabric medium ,because it will make the paint feel almost like a plastic coating..and that that will just sit on top of the leather and over time it will crack..You want it to be more flexible than just acrylics are that's what the fabric medium does to the paint BUT you do not want it to just sit on top of the leather you gotta do thin watered down layers.Like the consistency of thin watery cream.You only can put it on unfinished leather(no finish,oils or waxes put on it), Go for stuff like veg tan because chrome dyed leather or aniline already has a finish put on it.However you can get a tanned and dyed aniline or chrome tanned leather skin without finish on it ,it's sold as "crust".
Fabric mediums..for acrylics(I use the acrylics in little jars like folk art pure pigments etc. because they have less of the plasticizers in it,Tube acrylics aren't the best for leather they have too much viscosity)Here's a link to the kind I use Liquidtex..Dick Blick is pretty good on prices too.
http://www.dickblick.com/zz006/18i/Here are two kick ass places to get cool leather cheep
http://www.justleather.com/http://www.brettunsvillage.com/leather/Modeling tools
http://www.craftcave.com/leather/modelingtools.htmhttp://www.hobbytool.com/index.asp?PageAction=VIEWCATS&Category=3I love this site, they got stuff no one else has cheep And I mean can afford that on SSI cheap.I wouldn't have the shop I got if there was no science surplus!!Some of it's junk but some is not.You gotta know what you are looking for sometimes,anyways You gotta poke around here there is all kinds of stuff an artist can use here.They have the best little bead boxes ANYWHERE.I love the small aluminum multicolored cannisters that have lids and fit together in a box that comes with it.And they are great on dremel points,riffler files and other metalworking supply..Don't even get me started on the leds, submersible pumps and motors.
http://www.sciplus.com/One day I oughta compile all the links I got for art and crafts supply..I look around for cheep good and sometimes pretty weird stuff.
I got a really good source for glow in the dark pigments.In all sorts of colors, holographic films..I do some pretty weird stuff.