Heirlooms. At the bare minimum you make sure all the armor pieces upgraded to 110, since this is the lion's share of your experience increase. Neck and Ring upgrades are secondary, but still important due to being able to enchant them. Trinkets can't be enchanted, but you'll want them at max as well, if only to save time by not having to worry about replacing them as you level.
You can skill up your fishing from 1 to 150 in any fishable water in Zandalar or Kul Tiras. Go to the nearest water, find the fishing skill icon in your spell book (P) at the profession tab, drag this icon to somewhere on your action bar. Stand in front of the lake/river and press the fishing button. Mouse over the lure in the water, and when it splashes, right click on it.
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In the end leveling quickly in World of Warcraft is a lot about experience and leveling you 2nd or 3rd character will always be quicker than leveling your first one, no matter what leveling guide you use. We at FMG hope you find our World of Warcraft leveling guide useful and if you need help with anything or have comments about the guide, leave us a comment here and we’ll get back to you!
Edit 20 It’s been a week since this post and a lot of people are still reaching out to me with questions which is fine. I have had a few different people reach out to me to help make YouTube videos based off this guide and more info in BFA which I’m more than willing to do. Whoever gets the videos up first I will link them here for you guys to enjoy.