Step 1: Preparing the speaker.
But if you're as cheap as I am or have little money to spend on speakers, this repair is for you. So lets get started. The Victim for this job is a nice JBL 10" subwoofer. First, remove the speaker from the cabinet. Secondly, gently remove the old surround from the cone. About 90% of the speakers I find use foam. This stuff usually just crumbles away. This speaker is a bit nicer and uses rubber. This speaker as many others has thicker foam pieces glued over the top of the edge of the surround.They typically come out in several pieces. Use a knife of some sort to gently pry them off. Then pull the rest of the surround off.
The next step is probably the most tricky. Making the new surround. See if you have any old scraps of cloth laying around. The best cloth to use would be a synthetic blend, tightly woven, and smooth.I've used all sorts of cloth, including canvas.But you want to use something that's flexible enough to allow the cone to move freely. Use the speaker to trace the shape of the surround to fit. Secondly, measure how far in the surround reaches towards the center of the cone. Usually the surround glues to the very edge of the cone. But since you're using cloth, you will want a tad more holding power. So measure inwards about 1/8" from the outer edge of the cone. Use a compass to mark the inner part of the cloth circle to cut out.
Once you've cut the surround out, you will want to make slight compound cuts all along with edges of the cloth, about every 1/2" or so. You do this so the cloth won't crease or wrinkle as it is being installed.