What you need may depend on what the second battery is being isolated for. You certainly should have an isolator, but for some purposes that can be as simple as a big fat diode on a separate positive lead from the alternator. This would be quite adequate if the second battery is dedicated to something like a winch. A deep-cycle battery with its positive cable going only to the winch, and an isolated positive lead from the alternator would do just fine. For the occasional emergency boost, you could just get out the booster cables.
If you expect the second battery to be used for more than occasional cranking, you need more than that.
Years ago I had a Scout with a balky starter, and put in a dual battery setup. The positive side of battery 2 was connected to the alternator through a diode, and to the positive of battery 1 through a starter solenoid. When Battery 1 wasn't enough, I pushed the "magic button" on the dash, and got an instant boost.
If you ever owned a Scout 80 or 800, you'll understand why this was easier than replacing the starter. I eventually cut a hole in the floor to do the job.