Set-up: Fix a band to an immovable object and step inside of it. Make sure the band is not too strong - start with a light band at first.
Movement: Slowly and firmly step back one leg at a time, resisting the pull of the band. Do not allow the lower back to twist to the sides - focus on keeping your core tight and activating your glutes. Every two steps counts as a rep (one per leg). Once you have extended the full length of the band, slowly back walk to the start, and repeat as necessary. As long as the tension in the band remains challenging, these reps also count.