I've built a few boxes over the years. Here's where I disagree with them (their numbers):
2. I wouldn't use particle board as suggested. Either MDF or Birch Plywood.
8. Be very careful about any sanding after a cut. It's very easy to round over a flat surface needed for gluing, or introduce a curve by uneven sanding pressure.
10 & 11. I prefer an air-driven brad nailer to screws, but screws work OK (just more chance of splitting the wood if not properly pre-drilled & countersunk). After the wood glue has dried the screws and/or nails have little to do with box strength anyway.
14. If you use silicon caulk, make sure you wait about 24 hours before installing any woofer. Some speaker materials don't like the outgassing of the silicon as it dries.
17-21. If you're a beginner in woodworking, I'd recommend staying away from a terminal cup. Just drill two holes and uses threaded rods with nuts and washers (a dab of silicon couldn't hurt). Less chance of an air leak by sloppy fitment - many terminal cups don't have a very wide mounting lip.
One last thing. Probably want to figure out what you're going to cover the box with & get that done before installing the woofer.