You can create a new guide in a channel by clicking on the large + button (only members with sufficient access will see this):
It's important to note that administrators from a guides "home channel" have full admin rights, including editing, changing the author and access rights, and even deleting the guide. Therefore, take care to select the right "home channel" for guides you create.
Once you create the guide, it will be in "draft mode" until you click on "publish". While in draft mode, only you and administrators of a channel can see the guide.