I have foot calluses or corns. Can I cut them off myself?

Not adbisable. You can soften them sand them off no knives and be careful they are hard to shave off without injury even in trained hands you can see a podiatrist wear looser shoes.
Not safely. Don't cut, abrade. Use a pumice stone, or a tool intended for rubbing off corns, and don't go deep enough to start bleeding. Using scissors or a knife can cause infection, bleeding, or worse. Do not cut them yourself!