There are a few different ways that you can make a dado throat plate, but the most common and easiest way is to simply buy one. You can find these at most hardware stores or online retailers that sell woodworking supplies. Another option is to make your own, which is not as difficult as it may sound.
To make your own dado throat plate, start by measuring the width of your dado blade. Then, using a table saw or circular saw, cut a piece of plywood or hardboard that is slightly wider than the blade. Next, use a router to create a rabbet on one edge of the plate that is the same width as the blade. Finally, attach the plate to your saw's table using screws or bolts.
If you decide to buy a dado throat plate, be sure to get one that fits your particular saw model. And if you're making your own, take care to ensure that it is securely attached and will not move during use. With either type of throat plate in place, you'll be ready to tackle any dado-ing project!