Actually it is just wheat flour and water. Or you can use wallpaper paste. But it shouldn't be allowed to dry, at least not completely. It requires a polished piece of rosewood, boxwood or holly--something along those lines--or bone and the paste is applied and then rubbed until a glaze forms.
edited for punctuation and clarity
DW, pardon me for being ignorant, but where can I find wallpaper paste, and if I could, then how do I know if it is water based and are "natural" enough to use on leather?