That's going to depend on what your current wiring looks like. Mine was horrible. Not sure how much of it was from the factory or from the "professional" stereo installation that was commissioned by the PO but it was real bad. Non marine rated cable (copper, not tinned copper), multiple direct connections to the battery from the stereo and other options added by the PO made mine a candidate to completely pull it apart and start from scratch. I started a thread on my installation here....
The first step in determining what you'll need is to access the installation you currently have. How many wires are currently going directly to your battery? What kind of components were used? What options have been added over time that may not have been installed correctly? Ideally the only cables connected to the battery should be the ground cable to the boats main ground bus bar and the positive cable from the boat's main positive bus bar/breaker and starter circuit. The only exception to that would be for a direct fused connection to a bilge pump.
At a minimum you'll have to make up cables to connect the switch to the batteries, the grounds cables between the two batteries and the ground cable to what should be either a bus bar or terminal post for the boat's main electrical ground. Making cables is easy with some very basic tools (torch, solder, vise and heat gun for dressing the cable ends with shrink tubing) if you plan out your installation and get the right connectors and cable. A lot of us have ordered these components from a company called Genuine Dealz.....
For the money you'll save doing this yourself you can afford extra cable and connectors and probably do a much better job than most shops would do. If you don't want to learn how to make the cables Genuine Dealz will make them for you. You'll just need to mock up your installation and use something like a coat hanger to determine the path/length of the cables you'll need made up.
The key as with most projects is to take you time, plan out the installation and get the best materials possible to do the job right. There's always plenty of help here if you need it.