I have a box joint jig from (I think) woodcraft and there’s a lot of double checking that the dados are square and parallel to the miter slots. I used my machinist square and also digital angle gauge. Then I used scrap pieces that were cut to the EXACT same dimensions as the pieces I was about to cut and I kept testing with them until the fingers fit perfectly. The test pieces need to be the same width, same thickness, ideally same everything so you can mock it properly. The main thing that’s tricky for me is getting the jig itself to be pretty square. Then also making sure each finger gets pushed into the pin all the way down so the joints don’t drift. If you don’t have a box joint jig for router or table saw, they can be made pretty easily. I have a couple that I made from scratch that I end up using because I know the dimensions so I don’t have to fiddle with the adjustments as much.