Kitchen sink cabinets are the more involved installation processes of the flat pack kitchen. Firstly, the previous rough-in plumbing will have a vertical pipe coming either out of the floor or out of the wall to connect the sink drain plumbing to the drain trap. Also the hot and cold water copper pipes will be entering close by.
The kitchen sink cabinet is assembled (see my Kitchen Cabinet Installation page) and the drain pipe and water pipe entries carefully measured and holes cut for them with a hole saw drill bit.
With a helping hand, carefully place the carcass over the pipes and set it down for anchoring to the wall and the adjacent cabinet. After settling the carcass, caulk up around the piping to inhibit pest entry and limit leaked water movement.
The kitchen sink drain and drain trap are assemblies that require much forethought and personally, I like the trap to be under the shelf as shown below to increase usable storage space at a higher level. Installation of these items including kitchen sink faucets can't happen until both the kitchen bench-top and sink are installed.