I recommend using WoW Classic boost. You will receive not only the leveling up of the game, but also the fun of the game and an excellent team of like-minded people who will help you.
These guys saved my game. I am very grateful to them

In modern add-ons, spamming dungeons (or islands in Battle for Azeroth) is the most effective way to level up. However, due to the layout of the "vanilla" dungeons, and the difficulty of finding and saving groups for WoW Classic, this will not work. If you don't have high-level friends ready to guide you through the dungeons, you will have to limit yourself to a combination of quests with dungeons.

Luckily, WoW Classic is packed with quests, and if you take advantage of the XP bonus for resting, you can get to level 60 without ever grinding.