Click on "Get Codes (FANBOX)" or "Get Codes (PAYHIP)". It will take you to Fanbox (for JPY) or Payhip (for USD) where you can choose between three tiers that will grant you access to subscriber content.

Son Tier = Base art. No shota art.
Father Tier = Base art plus shota art.
Family Tier = Like "Father Tier" for people who want to tip!

Once you have the key, come back to this page and click on "Insert Codes".

And voilà! You successfully subscribed!