A guide is unlike any other form of academic or professional document. In fact, it's almost the polar opposite, so you need to take an inverted approach to writing.
Unlike most other documents, to write a guide, we recommend you start in the middle of a forest with nothing but an axe at your side. We're not building a road through the forest. We're clearcutting the entire forest, leaving only the biggest, and fattest trees behind in our path.
The deconstruction phase starts with cutting out everything that seems like 'theory' or 'ground work' for your topic. When you cut it all away, you should be left with only the key information that you absolutely have to get to your audience.
You're in the unique position of providing valuable information to an audience. The fact that the information can be DISTILLED to its absolute 'bear-necessities' is what makes the Guide format such a powerful tool. It enables you to get that valuable information to your audience in the most direct way possible.
***Most of what is above might best belong in Selection
The first thing you need to ask yourself is, "What do I know about", and "What do I have unique insights into"? Do you know how to prepare for a marathon? Can you provide insights into spending just 24-hours in Rome? Maybe it's something a little more complicated. Maybe, you know how to change the tires of a Sherman tank with nothing more than a paper clip and a pair of pliers. Identifying what you want to convey to your audience may seem minor, but having a clear topic is essential for creating a useful guide.
Once you know what your topic is, on a general level, you can start to refocus your attention on defining who your audience may be, and what you hope to show people with your guide.