Here is a guide on how to complete the Diamond Store heist without setting off any alarms. I have included a video demonstrating how the execute the plan but remember that each game has a variety of randomised elements which means you have to adapt the plan to your particular spawn.
Loot the specified number of bags of jewellery from the Diamond store.
The cases are all alarmed so before we breach the building and start taking hostages we need to disable the alarm and take out the cameras. Normally there is a side door to the security office on the left side of the building. Pick the lock and dominate or kill the guard inside. Check in this room for the keypad to disable the alarms - if it's not here it's probably going to be on the second floor.
The keypad looks like this:
Once the security room is under control, go around the back of the building and up the stairs to the rear entrance. Break into the back of the building, locate the keypad if you're missing it and locate the bank manager - she will routinely wander around upstairs and down on the shop floor, so wait for her upstairs.
With the keycard, disable the alarms and then prepare to storm the shop floor. Just before entering the shop floor, set up an ECM to block calls to the police and buy yourselves a little time to get the situation under control. Feel free to keep deploying ECMs for as long as it takes to get the civilians under control.
You will need one of your crew to head outside and control and tie civilians in the street and the owner of the shop next door. Remember that there is a courtyard out around the back of the diamond store and that civilians will sometimes wander around from there either via the side of the auto shop next door or the alley with the security room on it. Once outside is under control, this crew member can be used to move bags to the van - but always keep on the lookout for more civilians.
The remaining crew members inside should take care of all of the cases and drilling of safes if required. Bags should be thrown out onto the street to ensure the crew member outside can load them up in the van. Once you're done, get out of there.
Abilities I would recommend for this map are:
- somebody who can Dominate the guard in the security room,
- at least one crew member who can place ECM to block calls to the police,
- everybody with skills to increase movement speed with bags (Transport), interaction speed with bags (Fast Hands), etc
This map has been seen to spawn with very few guards, but in the event there are any guards that need to be dealt with, the usual skills apply for controlling the situation:
- at least one crew member with the Smoother Talker ability,
- at least one crew member with the Cleaner ability aced to dispose of bodies
Here is a video of the Diamond Store heist being completed without setting off any alarms: