You don’t need a bathtub. All you need is a bucket or a deep & wide utensil, 3/4th full with warm water..
Add 4-5 tablespoons of the hand/foot salt and stir gently so that the salt gets dissolved completely. You can also add a few drops of olive oil or coconut oil on the top of the water before putting your feet or hands inside. Soak for 15 to 20 minutes.