if you want spl, start out with a prefab box, they usually are pretty close to the recommend specs for the subs you're wanting to get...but if you want pure spl then one way to learn is download torres calculator, it'll give you all the dimensions for your box including port size. There are lots of info on building a box, i built mine living in an apartment, i cut all the pieces inside and glued/screwed them together and bought 2 4" aeroports.. Sounds good for the lows.