I would be interested to know about this too.... you might want to "emboss" the leather before highlighting it might be difficult to do fine detailing by hand. I have not seen the saddle in the pic, but suspect it was "embossed" (tooled) to get that level of detail.
There are multiple articles on how to dye "tooled" leather on the net along with some great videos.If applied correctly the leather dye will not rub off on you shorts etc.
Leather experts will chime in.
Ben