Here's how you can get started from https://tracetogether.gov.sg/:
After set up is complete: Keep Bluetooth turned on, and keep your app open in the background until the end of the outbreak.
TraceTogether runs in the background to keep you and those around you safe.
iOS devices / iPhones
After set up is complete: Keep Bluetooth turned on, and keep your app open in power saver mode when you're in meetings and crowded places.
This is because for iOS, TraceTogether works best in the foreground. To save battery, activate the in-app "Power Saver Mode” (see image above) to keep TraceTogether in the foreground with a dimmed screen while communicating with other TraceTogether-enabled devices.