https://glyphsapp.com/buy
Unzip the Glyphs app using the built-in macOS tool Archive Utility.
Move the extracted Glyphs 3.app folder to your macOS Applications folder.
Right-click the app and choose Open from the top of its context menu:
After double clicking the file provided by ECU, a dialog should appear:
For troubleshooting, please see https://glyphsapp.com/learn/how-to-install-your-glyphs-license