What the achievement asks
Put an Essence of rank 3 or higher into every slot of the Heart of Azeroth. Ten points.
Not account-wide, so it has to be done on one character.
It is the largest job in the Heart category
Every slot means the major slot and all of the minor ones, each holding a different Essence, each at rank 3 at least.
That is a wide net rather than a deep one. One Essence pushed to rank 4 does nothing for the other slots, so the work is finding several different Essences and getting each of them to rank 3.
The number of slots grows with the neck's level, so a character with a low Heart of Azeroth has fewer to fill. That is not a shortcut: the achievement checks the slots you actually have, and a fully levelled neck is the state most people are in.
Where rank 3 Essences come from
The sources are spread deliberately across the expansion's activities: reputations, raid bosses, dungeons, PvP, the Mechagon and Nazjatar content, and the patch 8.3 assaults.
Because the achievement only asks for rank 3 rather than rank 4, the practical approach is breadth: take whichever Essences your character is closest to on each source rather than committing to a particular set.
What has to be done first
- Have a Heart, the neck
- The Heart Forge, which is what makes the slots usable
- Power Is Beautiful is not a prerequisite, but its rank 4 Essence also satisfies one of the slots here, so doing it first costs nothing