Centauri's "The Definitive Guide to the Weapon Finish"
Recommended by Valve themselves during their "Dreams & Nightmares" skin contest, The Definitive Guide features 140+ pages of information about the skin making process. Great for both those just starting out, and well-seasoned designers.
The Definitive Guide to the Weapon Finish [overpay.guide]

Hollandje's "How to make your own skins for Counter Strike Global-Offensive"
Paired best with The Definitive Guide, Hollandje's eight part YouTube series covers the ins and outs of skin making, from downloading the programs to uploading your design to the Steam workshop.
Hollandje's Skin Making Tutorial

r/csworkshop Subreddit
Super useful community modded by workshop artists Quzga, Ezikyl_, and Hollandje. Definitely worth looking into if you use Reddit, it's a fantastic place to get your skin making questions answered!
r/csworkshop

My Discord server for workshop artists & skin enthusiasts!
Lots of helpful artists in the server that can help point you in the right direction when you run into a problem!
My Discord Server [discord.gg]

CSGO Workshop Discussions Page
CSGO Workshop Discussions

Valve's "Workshop Style Guide"
"Valve's Workshop Style Guide"

Valve's "Weapon Finishes Guide"
"Valve's Weapon Finishes Guide"

Valve's "Making Better Designs" Blog Post
Valve's "Making Better Designs" Blog Post

Valve's Workshop F.A.Q.
Valve's "Workshop F.A.Q."

CSGOstash
CSGOstash [csgostash.com]