The key to getting the true ending starts with choosing wei when you make your chapter 3 decision, and you'll want to focus on completing missions for cao cao's forces. When you get to wan castle, saving dian wei is crucial - clear out the enemy bases in both top corners, deal with those ambushing officers in the middle, and take down huche'er quickly, like within 8 minutes or so.

Moving on to mt. Bailang, keeping guo jia alive is your next big task. You'll need to work fast clearing those enemy bases, and make sure you deal with yuan shang and bailuan before they can cause too much trouble. The real challenge comes at chibi though, and this is where things get interesting. Make sure you've got guo jia as your companion, take down zhuge liang at that southwest altar, and when the winds start acting up, you'll need to find and defeat huang gai near cao cao's fleet. When you see that "fate altered" pop up, you'll know you're on the right track.

Just something worth noting - you can't get this ending on your first run through. You'll need to finish the regular story first, then tackle these special conditions in new game+ story mode. Once you manage all that, you'll unlock the true ending where cao cao's forces actually avoid that massive disaster at chibi, which is pretty cool when you think about how it changes everything.