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Before diving into DIY speaker enclosure design, you should pickup a few of the more well-regarded speaker design books (such as The Loudspeaker Design Cookbook by Vance Dickason, and Designing, Building, and Testing Your Own Speaker System with Projects by David Weems).
Some speaker building how-to publications are heavy on technical terms and math (it may take a couple reads before you fully understand all of the inner-workings of loudspeaker design). It is beneficial, however, to experiment with a few simple projects to get your feet wet. Mail-order/online companies such as Parts Express cater to the DIY market, and can supply the books, kits, and, materials you need to start your first project.