Below you can find profession leveling guides for every profession in World of Warcraft. These guides are up to date with the latest 9.0.2 patch.
Battle for Azeroth Guides
Old World Leveling Guides
Professions skills are split between expansions now in retail WoW, there is a separate skill bar for each. You will find guides below for each World of Warcraft expansion.
Classic WoW: You can find the guides for classic at my Classic World of Warcraft Guides page.
Fishing & Cooking