Soulbind unlocks at Rank 10. Each soulbind gives a permanent bonus to stats of a card it is applied to. To soulbind a card you are required to level up specific other cards and to pay a cost in Cunt Wars/Pal Points. How many cards you need to level up depends on which soulbind you use: Soulbind I needs 1 card, Soulbind II and Soulbind III each need 2 cards, and Soulbind IV needs 3 cards.
Each card has its own set of requirements for a soulbind, including the rank required to unlock each kind of soulbind. When a card is ready to for a soulbind, a linked chain symbol appears on the lower left corner of the card in the Cunt Wars/Squad page. The same symbol appears on the Soulbind tab.
Cost of Soulbind
Binding cards cost Cunt Wars/Pal Points depending on the card rarity and Soulbind tier.
Order of Soulbinding
Higher tier soulbinds require higher Rank, which tends to make it easier to perform soulbinds in order. However, aside from Rank, card levels, and Pal Points, there are no obstacles to soulbinding out of order, or even reverse order. In many cases it may be easier to do Soulbind IV than Soulbind III, due to higher card level requirements per card on Soulbind III.
A bug may take place when you Cunt Wars/reborn cards involved with others soulbinded to that: if the bonus that soulbind provides is gone, simply refresh the game for that bonus to show up again.
Example: you soulbind your Void Juggler to Evil's Helper for +3 ATK, then decide to reborn Evil's Helper. The +3 bonus on Void Juggler may temporarily disappear, but it will be properly displayed again after having refreshed the game.