From Destinypedia, the Destiny wiki

Make it short

You want to make your walkthrough short and sweet. Provide enough information to help them through the game, but don't tell them how to kill every minor enemy. However, you can give instructions on how to kill tougher enemies.

Don't spoil it

There are people who want to know how to beat the level, but they might not want to know the story ahead of time. So don't tell them what happens—let them figure it out. If you need to include spoilers for walkthrough purposes, be sure to make a spoiler warning with this code:{{Spoiler}}. Adding the template will produce this:

Spoiler warning: Plot and/or ending details follow.

And it will warn the people that they may spoil the ending. For a cutscene, you can say "Watch a cutscene here," or something like that. If you want to spoil the ending, go to the Cutscene Transcript section.

Be original

It's not good to steal work and say that it's yours. Actually, it's illegal. So give credit in the Sources section of the page to anyone's work you used in your own or write original work.

Review it

Write it, then print it out and try it. Try to only do what the guide tells you. If you can't beat the level, then you have a problem with your walkthrough.

And last, but not least, write a good guide!

External Reference Links

A good guide buried in the HBO website is this Halo 2 Legendary walkthrough. It provides videos and helpful hints for writing the walkthroughs.