OFF will do all the necessary things for creating a seperate CFS3 install. Just download it and follow the install instructions and a seperate CFS3 install will be automatically created and all the Over Flanders Field files automatically put there.
Nevertheless you shouldn`t need to use MultiCFS3 manually unless something goes wrong with the OFF install. MultiCFS3 is included with OFF but the normal installation will use it "behind the scenes" to set up the new CFS3 folder correctly. Only if this hasn`t worked would you need to use MultiCFS3 manually and probably not even then. A few people have found that OFF is still using the same user folders as the base CFS3 but in the cases I have seen MultiCFS3 has done its bit correctly but for some (as yet undicovered) reason CFS3 is just ignoring the new settings.