You're gonna need some tools, or a bike shop.

Pull the lock ring off, pull the cog off, install the freewheel.

You're gonna need a lock ring wrench, and a chain whip. I don't think you need a freewheel tool until you remove it.
I'd bet if you bought the freewheel at a shop, they'd install it for real cheap if not free.


Does your hub have threads on both sides?