Take a close look at your dreads every couple weeks to keep your 'do looking groomed. Not all dreads need to be tightened with your crochet hook; most dreads take about 1 inch of slack at the roots to "dread up" so there's no need fussing over that. Look for gaps further down in the dread, where your tight dread has come loose leaving a few free-floating fibers or developed a loophole. These raggedy buggers need to go to keep you looking sleek.
Slip your crochet hook into the dread at the loose part. The notch on the crochet hook should be above the loose part, while you've got the flat end in your hand, below the loose part.
Grab the loose hair and wrap it around the the crochet hook.
Pull the hook down through the loop in your dread, so you take the hair with you and work the hair into the dread. This is where it's gonna hurt a bit.
Pull the hook through the loop altogether, so it's free from the dread. The fibers in your dread will shift and tighten as you pull the dread through the loophole, leaving you with a tight cylinder and no more ratty dread!
Hook up all your dreads like this.
Using a crochet hook to maintain your dreads is a little ouchy. Since you're pulling a metal hook through your hair, there's no real way around it.
Grab a crochet hook at a craft store or yarn supply store and know that larger numbers indicate smaller hooks, which slip in and out of your dread with ease.