Repacking your silencer regularly is important to maintain power and reduce unwanted noise. Don't think that louder is better for power. Too much air past the holes in the core of the silencer will make your performance suffer.
The first step is to remove the silencer from the bike. Next take off the inlet cap by unscrewing the hardware or by drilling out the rivets. Don't be afraid of rivets, a rivet gun should cost about 10 bucks, and is really handy to have for lots of things. If the outer core is really snug on the cone, rap it with a rubber hammer to get it to come loose.
Next, pull the outside body of the silencer off the core and discard the old packing. This is a good time to analyze the condition of the packing, and compare to your notes on how many hours the bike has been ridden. Always take notes on your maintenance procedures. Clean out the inside of the silencer body with a paper towel, using contact cleaner if it is really dirty. Clean here means easier when you slide the new material in.
Take your new packing material, and lay it out flat on a work bench, a piece of newspaper on the ground, or even your seat. You might want to wear disposable gloves as this material can be itchy. Tightly wrap the packing around the core in a good firm roll. We like to secure the material for installation with two wraps of electrical tape. Other tapes might work, but electrical is durable against heat.
Reinstall the wrapped core into the silencer body. Slide it carefully, keeping the material in place on the core. Don't let it bunch up on the core. Now you are ready to secure the outer body to the cone with rivets or screws. When that is secure, you are ready to put the silencer back in it's place at the end of the line, and go riding.