Starting out with a rectangular lure blank is key.
If you start out with a rectangular blank, trace the profile of the lure, including the lip location, angle, and depth, onto one side. Then use a saw of come kind to cut the lip slot, keeping the blade square to the blank.
The easiest tool to use is a bandsaw, next is a scroll saw, or a hand saw. You can tape a couple of sections of hack saw blade together to make a saw that's the correct thickness for your bill, but leave a little room for epoxy.
If you want to use a table saw, you can use an auxiliary fence on you miter fence to set both the angle and depth, but it's a little more difficult. Do a Google search for miter fence and you'll see lots of examples.
Using a hand miter saw and a miter box works, too, if you make a wooden miter box and cut the angle of the bill slot into it first. You can cut multiple slot angles into it, and use whichever one works best for your lure.
If you are trying to add a lip slot after the lure is already shaped, a hand miter box is your safest option.
Mark a centerline from nose to tail on your lure, so you can set it into the miter box parallel to the side, and lock it there with wedges. Mark the depth you want the bill to go, and cut slowly to that depth, making sure your lure body doesn't move. If that sounds difficult, it is.
Getting the bill slot square to the centerline of the lure, and square at the bottom of the cut, helps you install the bill square and true, which is important.
If you trust your eye, you can set the bill into a sloppy cut and shim it with wedges (I use wood toothpicks) until it looks square to you, but that is an iffy proposition, and I only do it when I replace a broken bill, or try to turn a lure into a wake bait.
That's all I know about cutting slots for bills. I'm sure there are lots more ways, and someone here will chime in with other ideas, I'm sure.