How do I remove hard calluses on my feet?

Depends on the cause. So see doctor for evaluation. Make sure shoes are properly fitting. Can soak feet for a few minutes and use a foot file to gradually sand down the callous. Use a moisturizer on the foot to soften it.
Depends on the cause. Many things can cause calluses. Some reasons that one may develop a callus include: friction, pressure, repetitive trauma, and a limitation of joint motion. Some genetic conditions may also cause people to develop calluses on the hands and the feet. Treatment depends on the cause of the problem. Have it evaluated by a professional and get the appropriate treatment.
Gradually. You may try try emollients at night to soften the calluses. You could also use over the counter scrappers to gradually grind the calluses away. Avoid overdoing the grinding, it may take months to gradually wear away the calluses.