How to install complications on the Apple Watch.
- Take the Apple Watch
- Hard press on the display for e.g your Modular Face
- Tap on the "customize" button
- Tap on the complication you want to change
- Use the Digital Crown to find StepsApp in the list.
- Confirm by pressing the Digital Crown again
- Here is another useful link: Apple Support
To save battery life, the Apple Watch only refreshes complication data of third party apps every 10 minutes. If you want to view your live step count, just tap the complication to open StepsApp.