Expert answer
In 2012, What Works Clearinghouse approached a group of academics (including myself) as well as teachers to help them put together a practice guide for elementary grades in terms of what would be recommendations for teaching writing. These practice guides are evidenced-based but they are also based on expert opinion. And so the kinds of things that we found in terms of evidenced-based writing practices were the things that you might expect. One is that you need to teach process. Kids need to learn how to plan, revise, evaluate, and monitor what they are doing.
See: What Works Clearinghouse Practice Guide: Teaching Elementary School Students to Be Effective Writers
Second, you need to explicitly teach things like handwriting, spelling, and typing and help kids master the skills of becoming facile in terms of sentence construction. In terms of expert opinion, we also strongly believe that kids need to write for a considerable amount of time — at least an hour a day needs to be spent on writing and writing instruction. We also need to set up an environment to help young writers flourish — an environment that’s positive, where they see their teachers as writers, and where they work together with their peers.