Just as speech emerges, so does writing. Success at writing combines three aspects of development—fine motor skills, eye-hand coordination, and understanding language. Therefore, introduce activities in all of these areas as part of the emergent writing process.
Similar to the way speech (expressive) progresses at a slower rate than listening (receptive), so writing (expressive) develops at a slower rate than reading (receptive). Both expressive skills start later than their receptive counterparts. Then, even on their highest functioning level, well into adulthood, speaking and writing expertise never quite catch up with listening and reading.
In general, people understand on a higher level than they can express themselves verbally, and they read on a higher level than they can write. No matter. Reach for the stars in all aspects of learning.