Allow the child to choose topics to write about. Provide experiences that the child will want to write about. Allow opportunities for the child to just write down what they are thinking about. Ask questions that encourage the child to clarify what has been written. Don't expect the writing to be perfect from the start.
Writing is a complex process.
It takes time to master it. Allow the child to research an area of interest at the library. Then encourage them to take notes and write about what they have learned. Remember that writing is about communicating. Books for reading, paper for writing, and plenty of opportunities for reading and writing are the foundations for literacy.