The chief mandem is appointed by fellow mandems in a democratic process. The process involves a series of rap battles between the mandems which is followed by a vote of confidence to determine the winner and new chief mandem.
Stormzy: he just spat bare fire in that battle, I vote for him to become chief mandem
Skepta: yeah that man spits some mean bars I vote for him
Devilman: I'm too unpredictable so man don't want to vote
JME: by the power bestowed upon me by the gods of grime, I pronounce you mandem