Well, as some of you may remember, when I first got my two brothers exhaust I had a minor exhaust leak that I fixed with some high temp silicon. Well, at the time I fixed it, I lost the plastic tip at the end of the silicon tube and that made for a messy/poor repair.
Well I hope to help you newbs, like me learn from my mistake, and give you seasoned riders something to laugh at.
First, how to test for leaks. This is very simple, start your bike and run your hand were the slip-on and your headers meet. If you feel a rush of air, you have a leak, as simple as that...

well that's the poor job I did originally, you can see a lot of extra goo, and it didn't last very long.
Tools needed: Whatever your slip-on requires,(in my case 5mm and 6mm Allen wrenches, some high temp silicon sealant( I recommend spending a little more and getting the copper/higher temp sealant approx $6), scissors or knife to cut the tip of the sealant, and fine steel wool (if you need to remove old sealant) I used Aluminium Foil.
Why Aluminium Foil you ask?
1. I had some
2. I was to hungover to drive and get some
First things first, if you installed the slip-on, you know how to remove it. Remove your slip-on

As you can see, the bead of silicon isn't very consistent, hence the poor seal. This is why it is so important not to lose the plastic tip.
At this point take your foil or steel wool and scrub the crud off the headers and the sleeve. This makes for a better seal and also makes it look better.
If you didn't jack up like me, ignore this step.

Wow! Amazing! All is clean, don't forget to wipe any crud the might have fallen in the sleeve, just take a rag and swipe it through a few times.

Now apply you bead of silicon (about 1/4 inch) around your header, Don't go too low, as the sleeve will push the silicon down and give you too much silicon squirting out of the clamp were the headers and sleeve of slip-on meet.

Now isn't that better..

No go ahead and attach your slip-on. DO NOT CLAMP TOO TIGHT! This will negate all of your hard work, clamp down slowly until you start to see a little silicon come out, then stop. This gives the silicon a nice tight seal.

Don't forget to wipe!
Wipe your excess silicon off and LET SILICON DRY FOR AT LEAST 24 HOURS... This is no joke, do what the tube of silicon recommends for drying, trust me...
Once dried test for leaks again. Hopefully if you did it right, the problem has been solved, if not, grab some steel wool