Opening Superkeys for the first time

Superkeys isn't notarised by Apple yet, so macOS asks you to approve it once. It takes about a minute, and you only do it once: updates install themselves and never ask again.

1Click Done, not Move to Bin

The first time you open Superkeys, macOS says it was not opened. The highlighted button is Move to Bin; ignore it and click Done.

macOS dialog: “Superkeys” Not Opened, with Done and Move to Bin buttons

Already clicked Move to Bin? Reinstall it with brew reinstall --cask rxmeez/tap/superkeys and start again.

2Open Anyway

Open System Settings → Privacy & Security and scroll all the way to the bottom, to Security. Next to “Superkeys” was blocked to protect your Mac, click Open Anyway, confirm with Touch ID or your password, then click Open.

Privacy & Security, Security section: “Superkeys” was blocked to protect your Mac, with an Open Anyway button

3Turn on Accessibility

Superkeys needs Accessibility access to read Caps Lock and right ⌘ and to move windows. When it asks, go to Privacy & Security → Accessibility (called Device Control and Data Access on newer macOS) and switch Superkeys on.

Device Control and Data Access settings with the Superkeys switch turned on

Superkeys never records what you type. Keys pass straight through unless ✦ Caps Lock or ☾ right ⌘ is held, and nothing leaves your Mac.

That's it

The first time it opens, Superkeys walks you through the rest itself: turning on Accessibility, trying ✦ Caps Lock and ☾ right ⌘, and picking keys for the apps you use most. Take the tour again any time from Settings → General. From now on Superkeys keeps itself up to date in the background, with no more prompts.

If something doesn't work

The Superkeys switch is on, but the keys do nothing: macOS sometimes keeps an old entry. In the Accessibility list, select Superkeys, click − to remove it, then quit and reopen Superkeys and switch it on again.
I can't find Open Anyway: It only appears for about an hour after you try to open Superkeys. Open Superkeys again, click Done, then go straight to Privacy & Security and scroll to the bottom.
Removing Superkeys: brew uninstall --cask superkeys quits it, which puts Caps Lock and right ⌘ back to normal. Add --zap to delete its settings too.