Step 5Paint 'n stuff
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If you want to have shelves in your cabinet, secure those into place wherever you want them. The side walls on speakers are usually too thin to effectively use wood screws on, so if you're going to hold your speakers up with L brackets you'll need to either use nuts and bolts or some epoxy putty to hold them in place (I went with the epoxy putty, as you can see from the white globs beneath each of my shelves). Then again, you could just run some wood screws from the outside of the speaker box directly into the sides of your shelves, but I didn't think of that while I was doing the project, so I went with plan A (which, admittedly, probably should have been plan B).
Once everything structural is in place, lay on a couple coats of paint (unless you want to showcase the gorgeous particle board on the inside of your cabinet). I won't tell you how to properly administer paint, as anyone who doesn't know how to do that almost certainly wouldn't know how to read either.