Scientific writing is an important skill for a career as a professional astrophysicist. However, very few researchers receive any formal training in how to write scientific research papers of high quality in an efficient manner. This Perspective is the first of a two-part self-help guide to scientific writing to address this skills gap. This part focusses on planning your academic research paper in astronomy. We discuss how to crystallize the ideas that underlie a research project, analyse how the paper can be constructed considering the audience and the chosen journal, and give an overview of the publishing process. Whether you are a student writing your first paper or an experienced author, you may find the ideas presented here useful.