Okay, got your tools sorted? Let's do it... With a four-stroke silencer you should pack it from the end cap although some stock part packing (eg Kawasaki) can be bought as a single unit and placed inside from either end.

With a 5mm drill bit in place just simply drill the rivets out - if they start to spin and not cut give 'em a wee peel over from the side with a small punch to allow the drill to get some purchase. The end cap can be a bit of a bitch to get out - I use a Stanley blade to cut away as much of the sealant as possible and then get my heat gun on it to gently warm the puppy up. This tends to do the trick.

With the cap removed you need to use a hook to whip all the old packing out of the body and cap. Replace the packing in the end cap if required but don't pack it too tight, just use a nice even amount.

At this stage I check to make sure there's no damage to any of the internal parts of the pipe - if all's tickety-boo then proceed!
Tape the end of the core as shown - this stops you getting any packing where you don't need it and generally makes the job easier and less wasteful. Now get a good handful of your packing material and go all creative on it.

Make a doughnut shape (mmmmmm, doughnuts) and slip it into the pipe body.