It takes ten seconds. Lumen gets its own window and an icon in your taskbar, so it stops being a tab you lose.
On this computer
Click the three dots at the top right of your browser.
Hover over "Cast, Save, and Share".
Click "Install page as app", then confirm.
If "Install page as app" is greyed out, you are on a tab that is not a website, like a saved file or a settings page. Come back to this tab first and it will be black again. Some versions of Chrome word it "Create Shortcut" instead, and there you also want to tick "Open as window".
Turn on reminders
No problem! If you change your mind, you can allow them in your browser's settings.