Place the threaded part of your bolt into a vice, and tighten the vice so that the head of the bolt is at least a half inch exposed. This will give you a clear line of sight and space to work. Alternatively, clamp the bolt down on any work space, such as a shelf or counter in the garage. As long as the bolt is held in place, you are good to go.
Put on a long-sleeved shirt and all of your safety gear before you operate the tool. Attach the metal cutting attachment to your Dremel. Follow the guidelines for your specific tool. As a rule, most Dremel tools have a button that you hold on the neck as you unscrew or screw the head with your fingers to open or close the attachment connector.
Pull the trigger on your Dremel, and press the cutting attachment against the neck of the bolt, just behind the head. Apply slight pressure, and let the blade do the cutting. Go slowly so you don’t break or bend the cutting attachment. For best results, have the bolt positioned horizontally so that you can easily hold the cutting wheel against the neck.