If you're lucky, the broken bolt will be sticking out far enough that you can use a vise grip to remove it. If not, you'll need to drill a hole into the head of the bolt. To do this, first center punch the head of the bolt to create a starting point for your drill bit. Then use a small drill bit to make a pilot hole in the center of the punched mark. Once your pilot hole is started, switch to a larger drill bit and continue drilling until the diameter of the new hole is slightly larger than that of the broken bolt.
Now it's time to remove the broken bolt. First, insert a screwdriver or another tool into the drilled hole and turn it counterclockwise until you feel resistance. At this point, stop turning and apply pressure to the screwdriver while simultaneously tapping on its handle with a hammer. This will help loosen up any corrosion or debris that may be binding up the broken bolt. With some patience and perseverance, you should be able to back out the broken bolt using only these tools.