Once you’re in your Masters of Skinning, you will need to have your skinning level around 300 and then head to Hellfire Peninsula at the Outlands. The best place to skin in wow for Knothide Leather would be Nagrand. But beast there is around level 64 to 68 and will require skinning level of at least 320 to start skinning there. So stay at Hellfire Peninsula till you reach 320 before heading to Nagrand.
Once you have reached Nagrand you can go anywhere in the grass field to find beasts to skin. What worked well for me when skinning at Nagrand and around the water. It is always packed with beasts.
Hellfire Peninsula (290-315)(level 58-63)
1. Starving Helboar
2. Deranged Helboar
3. Thornfang Venomspitter, Thornfang Ravager
Nagrand (315-350) (level 64-67)
1. Talbuk Stag, Clefthoof
2. Wild Elekk, Bull Elekk, Banthar
Stay at Nagrand until skinning level of about 340 to 350 before moving on to Northrend.