All Steam guides must be attached to an App ID (i.e., the unique ID of a game or application on Steam).
For example, if you search for "Counter-Strike: Global Offensive" on Steam, you'll find this page:

https://psteamcommunity.yuanyoumao.com/app/730

As you can see from the URL, the App ID for Counter-Strike: Global Offensive is 730.

When you create a new guide, Steam redirects you to a link like this:

https://psteamcommunity.yuanyoumao.com/sharedfiles/editguide/?appid=730

This means your guide will be created under that specific game's page, in this case, Counter-Strike.

To create a guide not linked to any specific game, simply change the App ID in the URL to 753, which is a placeholder used for general (non-game-specific) guides.

Use the following link to do so:

https://psteamcommunity.yuanyoumao.com/sharedfiles/editguide/?appid=753

Once you open that link, you can write and publish your guide without it being tied to any particular game.