Where possible you should make sure you include instructions for every field inside a structure. This will minimise back and forth and clarify what content you need to get added to a field. Rather than having a single field called ‘text’ on an item, you might want to break it down to, for example, page title or product name.
You can then have specific guidelines for that piece of content. Things like character counts lock down how much content can actually be added and things like tone of voice instructions and links to external style guides can be really helpful to the author too.
Some people go further and include examples of how they want things to be written and/or links to examples of how their current site does things. As well as adding guidelines for specific fields in GatherContent you can also add Item wide guidelines to the Item notes section of a project. These allow you to upload files such as PDFs, style guides and this allows you to add purpose to the item.
Taking this further, a lot of people use tabs for content strategy notes per Item such as questionnaires or how an Item should be written. You can therefore add direction and ensure that all content you produce matches your strategy and your tone of voice.