One of my majors was in psychology, specifically toward the biological and neuroscientific side. I had to write many papers and there wasn't even one where I sat down and wrote it all out. I progress in bits and pieces, writing a nice opening statement in the introduction, a bit of content but not complete. Then I move onto the body and do the same. I review it, add, remove and re-write countless times.

My strategy for the introduction is to find the history and briefly discuss how the topic got from where it was to where it is. Professors love this as it allows you to include content from your course and gradually ease into the rest of the paper.

For the body, the main strategy is to make sure each paragraph is in tune with your arguments and introduction. I often ramble or get caught up in minute details, so I have to keep checking each time. When professors assess your writing skills, a large portion comes down to whether you use appropriate lexicon and whether each paragraph can transition from one to another.

When my paper is "finished", I give it to my parents or friends (as long as they study something else) to gauge whether a layperson can understand it. I then exchange papers with friends who do study the topic and see if it is not too simplified.

For your results section or whenever you include data, be sure to explain how the study got the values. The statistics can sometimes be convoluted (even as a person who took a minor in statistics) but you want to look for whether the actual data matches up with what the authors write. This is where you get lots of marks as it shows you understand the content and dug into it. It can also be the hardest part.