Okay. Well, first thing's first. Theme. It could either just be for teh lulz, or tie in and affect rules, etc.
Next, planning. Rules, stages, amount of time for sign-ups, amount of entrants, single or double elimination, prize or no prize, all that good stuff. This is the most important stuff. YOU NEED TO GET THIS DOWN TO THE POINT. I can't stress this enough. It's what screwed me over my first time around. x_x
Finally, the thread! Making a banner would be nice, but it's not necessary. Check out other people's tourney threads for examples.
After the sign-ups, you can either seed the brackets or make random match-ups. Once you have the match-ups, give people maybe two days to get their matches done. If they don't yet, message them with reminders. If they don't get their matches done after while anyways, give the match to whichever participant you want. If one is active and the other is not, give the match to the active one. Just remember this: try not to let the tourney drag on for too long. That makes people drop out and makes your tourney bomb.
Hope I helped.