Choose a route after completing the third class transfer.
The third transfer is required. Choose a route at the initial enchant and spend XP, SP and a Secret Book of Giants. Successful later enchants use XP and SP. Failure returns the skill to its unenchanted state; restarting needs another book.
Skill enchantment improves a chosen aspect of a learned skill. A power route and a cost route serve different goals: more damage per use or lower MP consumption. Check the routes offered for the exact skill before spending. A normal skill-level increase can change several values together, so it is not interchangeable with an enchant route.
Being eligible for the third profession is not enough: the enchant also needs spendable XP and SP. Inspect the displayed cost before each attempt. A failed attempt can erase earlier enchant progress, so decide how much progress and currency you are prepared to risk before starting a sequence.