Three days to make it real.

1. Tap “Download for Android”. The file is about 115 MB, so use wifi if you can. Chrome may warn that this type of file can harm your device — that warning appears for every app downloaded outside the Play Store. Choose Download anyway.
2. Open the file. Tap the download notification, or find tandem.apk in your Files app under Downloads and tap it.
3. Allow this one install. Android will say your phone is not allowed to install unknown apps from this source. Tap Settings, switch on Allow from this source, then press back. This permission applies only to the app you downloaded from, usually Chrome or Files.
4. Tap Install, wait a few seconds, then Open.
5. Sign up with your email. We send a link to confirm the address. Tap it and you are in. Check your spam folder if it has not arrived within a minute.
6. Say yes to notifications and location. Notifications tell you about matches and messages. Location is read only while the app is open, so people nearby show up first. Neither is required, but the app works better with them.

If the install button is greyed out: close the installer, open Settings → Apps → Special app access → Install unknown apps, choose the app you downloaded with, and allow it. Then tap the file again.

If nothing happens when you tap the file: the download may not have finished. Open Chrome → Downloads and check the file is complete, then tap it there.

Tandem is not in the Play Store yet, which is why Android asks these questions. Uninstall it like any other app: press and hold the icon, then Uninstall.