How to remove a callus on bottom of big toe?

It depends: A callus can be removed in many ways. It can be done conservatively (which affords temporary relief) or surgically. Calluses are caused by pressure against a bone. Debriding the thickened tissue will feel better, but as long as the pressure remains, the callus will recur. Taking the pressure off the area, via padding or an orthotic is beneficial. Surgical intervention corrects the problem.

See a foot doctor: A foot doctor can give you a accurate diagnosis to determine if it is callus and can the debride it if necessary.