Purpose of a report: writing to be read
A key thing to keep in mind is that a report is written to be read, by someone else. This is the central goal of report-writing.
Before starting writing a report need to have in mind the intended audience. In the narrowest of possibilities, your report is meant for reading by yourselves, and by your advisor/instructor, and perhaps by your evaluation committee. This has value, but only short-term. The next broader possibility is that your report is readable by your peers or your juniors down the line. This has greater value since someone else can continue on your work and improve it, or learn from your work. In the best case possibility, your report is of publishable quality. That is, readable and useful for the technical community in general.
Overall approach: top-down
Take a top-down approach to writing the report. This can proceed in roughly three stages of continual refinement of details.
First write the section-level outline,
Then the subsection-level outline, and
Then a paragraph-level outline.