Rats involves making and dealing a whole bunch of meth over three days, so prepare for a long job. For the first day, you'll have to cook a batch of the illegal substance.
First off, note that this job is almost impossible to perform with stealth. If you wish to try it, just keep low and keep moving. For the rest of us, this will be a "guns blazing" approach.
When you reach the lab, you'll find the cooks all dead. Take out any remaining Mendozas, and prepare to cook yourself. You'll find the ingredient at wherever they spawn within the house.
With ingredients in hand, begin cooking your first batch of dope. This is a good spot to hold out during the incoming police assault, so throw down Doctor and Ammo Bags and dig yourself in. Keep up the fight, and you'll soon see the cops retreat.
Cops may shut off power to the house during cooking, but this only lessens visibility. You can continue to cook without lights; bringing them back is nonessential, though they do help when locating the ingredients. Cook and haul your bags of dope to the van to wrap up the first day.