Your aim is to be read, understood, and remembered. Good writing is readable. It dose not require the rereading of every other sentences. It has clear transitions from paragraph to paragraph. Perhaps, clarity and accuracy are the foremost requirements for scientific writing.
Simple language is often the key to clarity.
Writing is difficult, but the best way to start is to start. Most people find that an outline is good place to start writing. The outline, like a map, tells how you will arrive at your destination, the completed report. The general rule is to use the past tense to describe events that occurred at a point in time prior to the writing of the report. The present tense is used to refer to assertions that continue to be true at the time the report is written.