How do I treat corns/ callous under my feet?

Severall things. First get some very good shoes and ones the right size and with very good support. Next, treat the corn and calluses with softening agents and wear padding over the areas in question. Next use an apporpriate emory, rasp, drimmel, or other device to pare, currette or file down the hard lesions. You may need special orthotics and to see a foot/ankle orthopedist or podiatrist. Correct defects.
Several options. . . A healthy, non-diabetic person can sand down his corns or calluses, which are thickened areas of "dead" skin cells. Self-care products are over-the-counter at the drugstore's foot-care section. The easiest way to get rid of them is to have a foot doctor, a dermatologist, or a primary care doctor shave them down with a blade (but such visits might not be covered by medical insurance).