It's hard to offer concise suggestions, but I would suggest as you review other modules to keep an eye out for whatever jumps out at you as "professional" and take note of what they're doing.
Graphic design is largely about organizing information visually, so think about what different types of content you have (price lists, game mechanics and stat blocks, narrative, Warden tips, etc.) and how you can visually distinguish them and let the reader know at a glance what goes together and what's something different.
RADSICK is a good example to look at.