Complete All Heists and Setups in Order:
This means that you must complete them in this order: Fleeca Job, Prison Break, Humane Labs, Series A Funding, Pacific Standard.
Complete All Heists and Setups with the Same Team:
You must complete them all with the same team of players from the beginning to the end.
Complete All Heists and Setups on Hard Difficulty:
All of the setups and finales, EVERY single one of them, must be completed in Hard Difficulty. Including the first mission where you have to drive with Lester to the Fleeca Bank.
Complete All Heists and Setups without anybody in the team dying:
If a single player dies while on the Mastermind Run, all the players will receive a pop-up message above the minimap, saying "Criminal Mastermind Challenge has been failed". So, nobody must die.


The Fleeca Job dilemma:
I get this asked a lot so I'm going to inform you now before I go into any further details.
The Fleeca job is a two-person job, but the rest of the missions are for four players.
The team has to split up in teams of two, which do the Fleeca Job together, or otherwise the Mastermind award will not count up for everyone.


I'll simplify it a little more:

Person A, B, C, D want to do the Criminal Mastermind challenge.
Person A and B do Fleeca Job together.
Person C and D do Fleeca Job together.
Person A, B, C, D do Prison Break after all that.