Perhaps the biggest beginner’s mistake in writing is to assume your audience knows a lot. You may feel like the least experienced person in your lab group, so anything you know, everyone knows. If you just mention an idea, others will know exactly what you are talking about. You feel you do not need to explain things everyone knows.
This assumption will make your paragraphs read like a disjointed set of topic sentences. Take your time. Decide for yourself exactly how the ideas connect and build the argument element by element, explaining the transitions. Is this easy? No, it is hard, because information is a net. You will always wish your audience knew A before you had to explain B, but then for A they should know C and D. Where to start? Some people write their main points on sticky paper or cards so they can arrange and rearrange until they find the best path through the concepts. Each will usually take a paragraph. Make sure you define carefully and avoid acronyms and initials. We do not want to learn your code. We do not want to learn your lab jargon. We read English, not necessarily as our first language.
Now that you know carefully building your argument is hard but important, you have met half the challenge. The best way to improve is to write a lot, ideally every day, even if for just 20 minutes. Pick something to explain and do it. Get a writing buddy and exchange texts to keep yourself going. You can also read a few pages of a favorite author, just to see how they build their case. For clarity, it is hard to beat Richard Dawkins.
Another common beginner’s mistake is to spend the most time on the methods. That is because this is what feels the most like the story of what you did. Take that energy about story telling and give it to the whole paper. We don’t only what to know what you did, we want to know why you did it. Happy writing!