You need:
- a fresh toon to "boost"
- a player in your lvl 1's group (can be anyone, any level)
- a lvl 60 hunter or warlock with a pet

1) Invite the second player to your fresh toon's group.
2) Place this second player/group member in the same zone as the toon you're "boosting", but keep him far away (for example in Elwynn Forest the player can stand outside SW gates while lvl 1 is in starting area, ref. the video OP linked).
3) Make sure you've got your hunter or warlock next to the boosted toon (hunter/lock should not be in your group).

After you've done the setup, tag a mob with your fresh toon and then have the hunter or warlock kill this mob with their pet. The mob needs to be killed solely by the pet, so the hunter/warlock shouldn't use any other ability than sending in their pet.

Repeat the tag -> kill process on all mobs you can find to get 100% xp from the tagged mobs.

Reference video Twitch

› See More: [WoW Classic] Mob tagging with a lvl 1 and lvl 60