Simply, obtain all the Plasmid Slots. All but one of these can be bought at the Gatherer's Garden. The final one is given to you as part of the story towards the end of the game and is unmissable.
If you don't want some pretty massive spoilers, stop reading now.
The final Plasmid Slot is given to you when you obtain the Plasmid to summon Eleanor Lamb after she becomes a Big Sister.