If you want to build your own subwoofer box, which is recommended to be honest its much easier and cheaper, but there's things you gotta know first. Depending on how many sub woofers your putting is the first thing you want to know. This also works if you don't want to make your own box and your going to just buy one pre-made you will want to make sure it matches up to the sub woofers specs.
First things first. Depending on if you buy your sub woofers brand new out of the box or used, you want to know what is the minimum/ recommended cubic feet. This is THE most important thing because this will determine how your sub outputs in performance. If your subs didn't come with a manual (used) than the best thing to do is make sure you have the size of them subs, the name of the company and everything else. For example I have JL subs and I have 12" subs and they are the W3V2(tells me on the back) and they are also 2 ohm load. (which doesn't matter I don't think) and just google it. something will pop up and tell you the specs.
Next you need to come up with measurements and see how well it'll match up to your sub woofers specifications.
For example I have a box which is 10in W x 42 in L x 14 in H and what I would do is multiply all those to get 5880, than you divide by 1728. Don't ask me why you divide by 1728 because I'm not sure why it's that number but whatever dimensions you have you will always use that number. Once you get that number than you'll divide by how many subs your putting in. I'm putting in 2 therefore I'd divide whatever number I got by 2. If its higher than your subs specifications, good. If it's below the subs cubic feet, don't expect maximum performance.
Once you got everything situated and got your measurements your ready to start making your box