If this has been covered previously, my apology to the originator. When I purchased a multi tool a few months ago one of the things in mind was to see if it would serve cutting hinge slots. With a current build in progress, it was time to see.

First - I took the flush cutting head that was 1.25 inches wide and having a straight saw blade edge to the grinder and ground it down on each side to leave a cutting portion the width and depth of the hinges I use (Dubro #117). Keep it from getting too hot by dipping occasionally in water. A test piece revealed that the cutting teeth (at least on the Harbor Freight version) provided just the right thickness of cut for the hinges. In the test I also noted that the slowest speed on the multi tool worked best... so if a multi tool is in your future and you might use it for hinge slotting, I'd suggest a variable speed.

Second - I clamped a piece of oak hardwood to the bench edge to serve as a base reference for the cutting head. The hardwood strip needs to be positioned (shimmed up) to the approximate height of the hinge slot desired in the part that will lay just beyond the strip. Keep this simple with some spring clamps so that the shim can be changed easily as various height needs change.

Third - Lay a towel beyond the hardwood strip and place the surface to be slotted on the towel and bring it up to the hardwood strip. It can be rotated slightly to get the right height or the shim under the hardwood can be changed.

Four - With one hand, hold the part to be slotted to the hardwood strip and at the right height and rest the cutting tool on the hardwood strip and advance until to the depth of the shoulders on the cutting tool. Easy and fast. I made 38 slots in just a few minutes whereas by hand I'd been at it a couple of hours.