For any general game:
-Grammar, spelling and mechanics. Liberal use of the enter key. Something that isn't a total eyesore, hopefully.
-Clarity. Don't say "get the C4 plant the C4 run to the subway." Tell them how to do it. It's kinda why people read walls of text such as these ;)
-NO spoilers. This will be the one section where spoilers are a no-no. Obviously you'll have to talk a bit about the plot, but you should NOT mention anything coming up in the game. ONLY things from the beginning of the game you're walking through to the level you're in are allowed. And don't spoil the mission itself either. I would hate if someone said "When you hear the gunshot after Murakawa kills himself run in and open the secret door" and I had NOT even beaten Tokyo yet ;)
-A mission outline at the beginning of the level's walkthrough, including all upcoming objectives. Filter the name of the objective to avoid spoilers, if you need to, ie: "DPE on the Agency mole" instead of "Kill Benton" for the museum in SF1. Also, for OS, if the objective is (hidden) or (online), mark it as such. Maybe numbering or lettering the objectives will help for the next point.
-Mention when people have completed an objective. All the great walks have 'em. Something like:
Objective A Complete!
For Omega Strain:
-Par-times should be earnable by your guide, so make sure yours is spiffy!
-If an achievable offline objective happens to be in your level, don't confuse the reader by smacking it in the middle of your guide. Especially since some of them, like saving the SWAT officer in Carthage 3, make par impossible. Put it as a footnote at the bottom.
-A recommended loadout is good, but don't INSIST a gun if it isn't necessary. Instead, put OPEN and tell them what KIND of gun they want, like something with IR telescopics for example, to see through Myanmar's mist. And if a specific gun is necessary, tell them why (sans spoilers).
-Reasons for your recommended loadout, so the reader can make a substitution that still fits
-All objectives, hidden and so on, revealed and in the most "professional" way available to the player (like RESCUING Dobranski in Belarus instead of just blasting his safe)
-I didn't, because I suck, but ideally you want an objectives list at the start and notations of when you've achieved what objective in the walkthrough. And such.
