What is the adverb for leather?

We do not currently know of any adverbs for leather.

Using available adjectives, one could potentially construct nonstandard adverbs such as leatherly, leatherboundly, leatheredly, leatherierly, leatheriestly, leatherlessly, leatherlikely, leathernly, leatherily or leatheringly.

Note, however, that these words will not generally be contained within standard English language lexicons.