Dip a shank of a blank skeleton key into hot wax and then let the wax fully dry onto the key.
Slide the waxed key into the lock. The wards inside the locking mechanism will make indentations on the waxed key, which will indicate where you need to remove metal from the key blank so the key will work in the lock. Pull the key out of the lock
File metal off the wax-covered key blank where you see indentations on the wax. The indentations indicate that portion of the key struck the wards when you tried to turn the key in the lock. File off all the metal that hit the wards. File right through the wax -- it will act as a lubricant to help you cut. To ensure the key passes over the wards in the locking mechanism, you can file slightly past the marks on the wax. This will provide additional room for the key to pass the wards .