At the beginning of the Counterfeit heist, the host shows you a leak to fix in his basement. Right after that, he'll get back to his guests, leaving you alone like the poor poolboy you are supposed to be. From that moment, is it possible to make your first steps into the heist undetected, delaying the first cops hordes ?
According to a Wiki you have to do the following:
Prior to starting the heist, the crew is advised to spread out and find all 7 or 8 civilians, who are in Mitchell's kitchen, the garden, and sometimes Wilson's house. There will sometimes be civilians on the far side of the garden, and the girl in the bikini is sometimes in the lounge of Wilson's house.
Prevent all citizens from escaping when the heist starts, and restrain them with cable ties. You may have to kill some civilians if you are playing solo and they are spread out too much. Once the civilians are under control, set up a hacking device in Mitchell's basement, then move the bookshelf in Wilson's basement and start hacking the revealed code lock.
Before any cops arrive, take the opportunity to search the houses, garden and garages for a crowbar, which is required for future objectives. Exactly two bars appear in each playthrough, and at two of the following locations:
Wilson's garage, by the car's rear right tire Partially underneath the front of Wilson's bed Mitchell's garage, in the middle Mitchell's bedroom, inside a shopping bag near the bathroom Right next to the pool The shed in the far corner of the garden Below the rear of Wilson's boat
Be sure you are not seen by the neighbors. Once the crowbars are found, dedicate one crew member (preferably with a silenced weapon) to silently neutralize incoming cops, and the others to restarting the hacking devices as soon as possible.
If the crew manages to stay undetected, Bain will mention that someone must've seen something and that two officers are coming to check out the address. The crew must stay hidden (perhaps at the top of Wilson's staircase) and wait until Bain says the officers are inside before killing them with a silenced weapon or melee attack, or taking them hostage. If successful, the crew will remain undetected.
After a while, the police will call Wilson's telephone number, but the crew cannot pick it up. Two more officers will soon enter the garden through the side entrance. Remain hidden again (the top of Wilson's staircase works here too) and silently neutralize them once Bain gives you the cue. Shortly after, the police officers will be reported missing and SWAT will begin to arrive.
If you fail to remain undetected, then law enforcers will come as soon they are aware of the crew's operation. Units will be coming from predefined points of the map, jumping over fences and walls, and sometimes a helicopter will hover above Mitchell's house.
Found some videos, that should help