Give them freedom and encouragement to write and revise a number of times without fear of failure, and grade only the final, finished product. Even then, don't mark up every line. Instead, praise a few things the student did very well, question a few that didn't work, and suggest two or three ways to improve the paper. The results will far exceed what you can accomplish by spilling a lot of red ink. Do this above all.
Writing is a skilled craft.
Do whatever you can to keep them writing every day in ways that also get them to think about your discipline. Have them turn their writing in to you from time to time, so that you encourage them to stay at it, but, as I said earlier, don't correct and grade everything they write. Give them space to learn. No fact about writing is better established than that people who do not read often and well do not write well. The relation is causal, not coincidental.