I hope this helps explain, for all those who continue to ask how the heck one resonates their armor.
And remember: you must imbue the piece of armor you intend to resonate to +6 before you can do this.
Also: you will not recieve the skill bonuses unless all 3 pieces of armor are of the same elemental resonation (i.e. erda, etc).
Edited to add: Even with the ancestral crystal, there is still a chance that the resonation will fail and your armor will be destroyed. Although, if you use the crystal, the chance of failure is very low.
Credits go to Illeana.