Using a drill with a handle will help keep the hole square. The hole needs to be good and straight for the new plug to seal.
You can also buy a bushing with a good flush end and drill thru it while keeping it tight against the case to help keep the hole straight.
Helicoiled threads have to be tapped straight with the sealing surface to allow a 360º
seal underneath the plug head or the plug will leak. 3)4)
Before drilling, the hole was packed with grease and a small piece of stainless was put on the other side to keep from drilling
anything and (hopefully) stopping some of the chips from falling further into the engine. 5)