The Duggan Robbery has two mandatory tasks and one optional task.
Task 1: LS Panic Trailer
In this task, you need to steal the LS Panic trailer. When you go to the location and start the task, you will be asked to break into a warehouse.
When you get inside, eliminate all the guards and be very careful of the guard with a shotgun.
Once you've eliminated all the guards inside, try to enter the LS Panic trailer. You will get a message that it is locked and that you must find the cab keys.
The cab keys are found inside the warehouse office. The exact location of the cab keys inside the office is always random (table, shelf, etc.), but it is always inside the office.
Once you've got the cab keys, return to the truck and enter.
When you exit the warehouse, you will have a 2-star wanted level. You need to lose the cops now.
After losing the cops, deliver the LS Panic trailer to the marked location.
Task 2: LS Panic Outfits
This task is an easy one. When you start it, you must go to the clothing store.
When you enter the clothing store, you need to look for a purple duffle bag with the LS Panic outfits. The duffle bag is usually on the ground behind the cash counter.
Once you've picked up the outfits and exited the clothing store, you will get a 2-star wanted level.
Lose the cops and deliver the outfits to the Salvage Yard to complete this mission.
Task 3: Masks (Optional)
Masks are optional and only add an "aesthetic" to the whole mission. Whether you get the masks or not, it won't affect your robbery in any way, shape, or form.
Plus, if you want a free set of masks for this robbery, you can get one from the mask shop at Vespucci Beach.
With these initial tasks completed, you are now ready to proceed to the more involved Planning Work missions.