Install the app on your phone. It will count your 'steps' with phone's sensors or through Fibit integration if you use one. Make daily reports with your app. That's it :)

Here are many reports by happy users: https://steemit.com/created/actifit
And here is the latest @actifit daily status report: https://steemit.com/actifit/@actifit/actifit-updates-top-daily-actifitters-doubled-up-winner-delegation-status

Good luck!