Stick with whatever prongs the tool has.

If the tool has a two-prong plug, use a two-prong cord. If it has three prongs, use a three-prong cord. That’s it.

The whole point of grounding is safety if something shorts inside the tool. Old two-prong tools are either double-insulated (the common reason they don’t have a ground) or they predate grounding requirements. Either way, adding a ground prong to the cord doesn’t help if the tool isn’t designed to use it. You’d need to modify the tool itself, which is not a beginner project.

For extension cords specifically: never cut off the ground prong on a three-prong cord to fit a two-prong tool. Just buy a two-prong cord. They’re cheap. And never use a cheater plug (three-to-two adapter) unless the outlet box is actually grounded—which it almost never is in old houses.

Older tools are simpler than we think. Don’t overcomplicate the cord.