Download and run the KeeForm installer. The defaults are fine for most users. You only need to do this once, even if you install KeeForm on multiple browsers.
Edge users: first enable Allow extensions from other stores in Edge Settings → Extensions.
If KeeForm stops launching Edge, disable Startup Boost in edge://settings/system.
After installing, make sure the extension is enabled — KeeForm shows a green indicator when everything is working. From now on, double-clicking am https URL in KeePass opens the browser and fills your login automatically.