Red Dead Redemption does not do a very good job at teaching people how to duel. It is not intuitive and that leaves many people confused because it's not easy to practice.
Once you accept a duel:
In the duel, once you see the words "DRAW!" Press and hold LT(360) or L2(PS3).
With LT or L2 held down, move the right stick, you should see a red cross hair that sometimes flashes white.
Move the cross hair to where you want to hit and only pull the trigger (RT for 360 or R2 for PS3) when the cross hair flashes white. Do not pull the trigger when the cross hair is still red.
Keep shooting the trigger when the cross hair is white to fill up the blue bar on your lower right. Once Dead Eye is over and if your blue bar is bigger than the opponent's red bar, you'll win the duel.
That's all there is too it.
- For extra +100 fame and +200 honor, do not kill the opponent, just disarm them. Aim for the gun and you'll be able to pull this off. If you do this over and over (eg. cheating at cards so you force a duel) you can build your fame extremely fast. Note: you can't do this with most story driven duels, you have to kill.
- There is a random dueling event where a guy stands there dying to pick a fight with you. In Bonnie's ranch he's just outside of Bonnie's house by the bridge, you'll see him as a blue dot in your map once you get close enough. In Armadillo, he's by the wanted posters. Search for him whenever you're in town to build points in fame and honor.
- The more famous you are, the more people will challenge you to a duel.
- Do not pay attention to the blue and red bars until you've landed most of your shots. It's a distraction that takes away time from aiming properly in a duel. Concentrate on marking your shots as fast as possible and your blue bar should be able to beat the opponent's almost every time.
- There are people that say you don't have to use LT/L2 to start aiming but to just start using the right stick and the cross hair will appear. I'll test this later and update this post if it works.