Install Notate
Notate runs on desktop, mobile, and the web. The desktop and mobile apps are free and work offline in Guest mode, with no account required; the web app needs an account with Sync.
Desktop
Download the app for macOS or Windows from the download page. Run the installer and launch Notate. On first run you can point it at a folder of markdown files you already have, or start a fresh one.
Mobile
Notate is available for Android and iOS. On Android your notes are stored through the system file picker; on iOS they live in the Files app. You can start writing immediately in Guest mode, with no account required.
Web
The web app runs in your browser with nothing to install. Because it has no local disk to write to, the web app requires an account with Sync turned on.
Next: create your first note.