Certified GKH Gardening Expert
You don't, actually, have to do much. this plant (unless it is a common perennial), only lives for two years before it sets seed and dies. If you really want to cut it back, I would suggest letting it die off naturally first. Then, while it is in dormancy, it is safe to cut it back.
Here is an article that will help you gain more knowledge of the plant: https://www.gardeningknowhow.com/ornamental/flowers/foxglove/foxglove-winter-care.htm