Check the wattage required to run your piece of musical equipment. In the case of amplifiers, this is not the output wattage, but the input wattage. In order to find the input wattage, you may need to look on the back of your amplifier (generally underneath the voltage marking) or in the owner's manual.
Purchase a voltage converter that is designed to transform 110V to 240V. Make sure it is converting the voltage in the correct manner (from 110V wall socket to a 240V supply, not the other way around) and that it can handle at least as much wattage as your musical equipment requires to run. If you purchase a converter that is too weak, it can explode.
Plug the converter into an electrical socket, and then plug the musical equipment into the converter.
Turn on the converter and then turn on the musical equipment. The equipment should work fine now.