A toe at a time.
Heck, why not start with the nails?
Starting with a very slow removal of each toe nail first by an angry podiatrist - while Dzhokhar Tsarnaev watches. Then start with the toe amputations sans anesthesia, pain medications, and anesthetics. When he passes out, wait for him to come to before continuing. Then continue with the leg amputations under the same conditions. Do these procedures in his cell with no sterilization procedures or conditions.