The best way to do this is to use a hole saw bit that's the same size as the existing hole. Using a piece of scrap wood, cut a plug and take out of the saw and fix it on the pilot part of the larger saw. You must ensure that the plug goes below the teeth of the saw. Pop it in the hole that you want to make bigger and start drilling. The plug will remain secure because as it twists, it will screw further into the plug. This way the whole is enlarged, job done! It's useful to bear in mind that a hole saw only has teeth of the perimeter, you aren't able to enlarge the hole in the guide jig. But you must align the guide hole and power drill axis together, otherwise there is a risk that the jig will get damaged. If you have clamps with a quick-release function, then use them to keep the guide in the right place before moving on to the next area. For longevity, you could use a piece of metal as the guide. Wood is okay but won't last as long and metal is far easier to clamp down with. You can also use a drill press. Conclusion So, that’s everything you need to know on using a hole saw on drywall. Happy DIYing everyone!