With all switches powered off and links disconnected, connect the switches together via stacking cables in a ring topology. To create a full ring, start by connecting switch 1/stack port 1 to switch 2/stack port 2, then switch 2/stack port 1 to switch 3/stack port 2 and so forth, with the bottom switch connecting to the top switch to complete the ring.
Before adding it to the stack, add the switch to the network, plug it in, and leave it for 24 hours. They often spend 20 to 40 minutes when you first bring them online doing firmware updates. Otherwise if you just add it brand new to the stack, it might look like the stack is not working for 20 or 40 minutes.
There is a known bug which occurs when you disconnect stacking cables and then connect them back, it might hit you:
"Stack members may go offline after a stacking cable is reconnected requiring a power cycle (present since MS 12)"
We hit these way before it has been reported as a known issue and rebooting one of the switches in the stack most of the time helped, but just expect running into issues, so if you don't want that, then power down the whole stack.