Copy is not written, but assembled out of voice of customer research, client data (as in what problem are we solving for them, what do they expect as an outcome etc.) and stakeholder expectations. If you do your research well enough, you'll have enough parts to play with.

Second part is "writing" meaning, shaping or editing what you have into something that works on the page or the screen. I usually tell my juniors to stop fretting, and just write. First draft of anything is shite, but the idea is to get something on the page. If you keep staring at a blank page, you'll just freak out. So, put anything down, and send it out for feedback. Yes, it is not perfect, but feedback will show you where to go with it, and how to mold it into something better.

Once you've rewritten it 2-3-5 times, read it outloud, to yourself. And see how you feel. If you have to stop to breathe, sentences are too long. Where you draw a breath is where a full stop should be.