Taking hides (whether dhide, cowhide, or other) to a tanner (in Al Kharid, Crafting Guild, Ranging guild, Canifis or Neitiznot).
You pay a small fee and he makes it into leather, which you can use for crafting.
Obviously, tanned leather sells for more than the raw hides... and you can make a nice pile of money after doing it for a while.
I personally made ~12m on a noob of mine by tanning. It wasn't fun but it got me the money I wanted.