For the insert nuts I used in the video, they have an outer diameter of about 8.5mm. For this type of nut, a 9mm pilot hole should be fine. For softwood, you can even use an 8mm pilot hole to achieve a very tight fit. Always follow the manufacturer's recommendations for pilot hole size. You can use a depth stop on your drill to help determine how deep to drill - this is especially handy if you have a lot of drill bits to fit.
Fitting a screw into a hole can be tricky. Some screws come with a built-in hex socket. So, have a slot designed for a flat-head screwdriver. A simpler solution is to use the machine screw you plan to use in the final installation and install a locking nut to prevent it from turning. You can then use an impact driver to thread the insert nut all the way in. Loosen the locking nut with a small wrench, and then you can remove the machine screw. This is also a method you can use on headless threaded inserts.
I prefer to use carbon steel insert nuts because they are strong and durable and less likely to break. Obviously, if the fitting will be subject to any moisture, make sure you use a corrosion-resistant nut. You can buy brass insert nuts, but generally these are only suitable for very soft materials or they will likely break when inserted.
