This is more complicated than it sounds.
1st: You need to pick a game with more than 20 minutes of content.
2nd: You need a game with enough story and/or gameplay to require a guide.
3rd: It should always have at least three points, as it sounds more complete that way. Like you were actually trying.

Take your chosen game, making sure it has the previously stated aspects, and play it.
While playing, remember important features. Here are a few in most games:
1st: Movement. Movement is in many game and you have to mess with something to do it, you could write about that.
2nd: Story. Some story plots are hard, help out your fellow man by explaining the obvious. I know what you are thinking, but it's not subjective, everyone else is just wrong.
3rd: Gameplay. Do you shoot guns, stab monsters, hit enter repeatedly? It's perhaps the most important question of all. Answer that question you genius, make everyone recognize your brilliance.

Remember all that research? Use it like you'll lose it! Or repeat step 2 if you lost it already.
Here are some tips:
1st: Grammer: If they can't read your guide it's either a gag or in the wrong language. Always go for the second option if you want to look serious.
2nd: Sarcasm: It's unnecessary, don't go down that road. It's not worth the price you'll pay when people facepalm themselves to death.
3rd: I don't know, imagine something inspirational.

Carpe Diem is great. I also really just wanted to make a guide where it didn't apply.