Mutagen slots can be found on the character development page. There are four slots, each one unlocking when you reach a certain level, the last one unlocking at level 28.
The mutagens that you put in the slots can be found on the bodies of your enemies and can also be crafted in the alchemy menu.
Matching the skills that are placed in the three squares to the colour of the mutagen that you have slotted into that area will also increase the power of said mutagen (I.e. a 10% sign intensity mutagen will increase to 40% if three matching colour skills are slotted next to it.