First, you will need to complete everything in the To Do list and then complete all the tasks in the Optional List except for the Duggan Shipments and the Power Drills.
However, the fastest and easiest approach to complete the heist without alerting anyone is the Big Con Approach. You can complete the Diamond Casino Heist using three strategies: Best Diamond Casino Heist Approach – GTA 5 Online This guide will tell you about the best approach to complete the Diamond Casino Heist. There are mainly three ways to complete the Heist and each one of them offers different rewards and payouts depending on how well you manage to execute them with your teammates. In addition to that, there will also be a 50% reduction in the overall setup cost which makes it the perfect time to take part in the Heist. Rockstar has announced that it will offer 50% more cash and reputation points for players who complete the Casino Heist this week.
GTA Online is constantly updated with new outfits, events, weapons and heists to keep players engaged.