Make it UNDERSTANDABLE and INTERESTING TO READ !
Usually, i read the whole manual before playing a game...that's what i tried to do with Tremulous...but i couldn t.
The previous manual is full of "Use Button X for Special Action Y" (with no explanation/example of Action Y) for every class...you cant understand it if you didn t play the game. I couldn t understand, i couldn t remember, so i dropped it, played a few games, and used only the tutorial mode to learn the commands...i completely forgot the crappy manual. That's why it took me quite a long time to learn about sprint, stamina bar, etc...
Actually, i wish we could ask someone who never played the game to write the manual...I know it sounds silly, but an old player cant notice how hard it is for a beginner to grab the concept of alien classes and all the abilities players can use.
I wanted to try to make a new manual, but i couldn t find enough time for that.