I got one of these, not this exactly but this type (knife type kill switch). Mount it on the NEGATIVE post and mount everything that was on the negative post onto this switch’s post. Make sure you have every single wire that goes to that negative post removed and out onto the switch, otherwise a number of things will bypass the switch.
WirthCo 20138 Battery Doctor Battery Disconnect for Top Terminal https://www.amazon.com/dp/B000CQBND0..._CcZ5Cb7MQ9JBF
Heck your battery type though because some have top posts and some have side posts and some have other kinds of connectors.
I had very bad luck with one of those brass “screw-on, screw-off” type switches. The two parts kept splitting apart causing arcing and it was not a good situation! (This type - no! Docooler Battery Terminal Link Switch Quick Cut-off Disconnect Car Truck Auto Vehicle Parts (Battery switch 1) https://www.amazon.com/dp/B00GJMDTH4..._GhZ5Cb6NSM4KG