I have my students do toe stretches before we begin the core practice.
1 With your feet together, toes spread as wide as you can, bend at the knees and bring your hands to the floor in front of you (like you are checkin out something on the ground). Lean forward on to your hands, allowing your heels and balls of your feet come off the floor. The further you lean forward, the deeper the toe stretch.
2 Come back to seated, lace your fingers between your toes and roll your hands/feet around in circles both ways, and front to back.
3 Press your thumbs down the soles of your feet, especially right in front of the base of your heel.
4 In plank position, send your energy through the back of your heels and then slide yourself forward and back, actively stretching the heels and toes.
5 Stick to less intense modifications where you can to minimize injury/strain. Maybe focus on supine and seated poses for awhile until your feet feel better.