This foot warmup focuses on foot articulation and rotation of the hip, and is a great way to start any barre (or center barre if that's your jam!)
Make sure that when you do the flex and plié that the hips stay square, that you really turn out both legs, and that the tailbone stays pointed down towards the ground. It's easy to "over-tuck" in this position. It's also easy to tilt to the side of the working leg.
You always have more turnout in plié than you do with straight legs. Take advantage of that! (but do watch your knees, and make sure your turnout is coming from the hips, not the feet or knees)
This song goes on forever. Feel free to add more stuff to it to fill it up.
This combo is from my Disney Highlights Intermediate barre!
From 1st, facing the barre;
1: tendu right foot front
2: flex the foot and plié
3: stretch and tendu
5: tendu right foot front
6: plié in 4th position
Repeat with the left foot, then to the side and back.