Now I am by no means the most learned person on this topic, and others have tackled this subject with more detail and eloquence than me*, but I will do my best to provide a broad overview of how the Charlotte Mason method teaches children to write.
Firstly, younger children learn to form their letters correctly, focusing on quality over quantity. As they become more competent, they will write whole words and whole sentences. Children are always encouraged to write in their best handwriting, but for no more than 10 minutes.
Unlike in schools today, young children are not encouraged to compose original written work; instead they are asked to copy a piece of excellent writing, paying careful attention to spelling and punctuation as they go. In this way children are focusing their powers of concentration on what great writing looks like and aren’t expected to make the many multiple decisions required when one is thinking up something new or trying to decide what they want to say.
Children are encouraged to express their unique ideas and perspectives, but in the younger years this is done orally, and is normally a narration – a ‘telling back in their own words’ - of what they just read or listened to.
Children would always be encouraged to copy writing that is interesting and engaging and full of ideas worth pondering on. Ideally this would include texts from books the children are reading. (I tried to do this but it became a logistical headache with four children in my house!)
As children develop their writing stamina and maturity, they copy more complex works of writing and, as they approach the age of 10 or 11, begin to compose their own original pieces of writing, a little at a time. By this age, simply through the process of regular copywork over several years, children have internalised correct spelling for a wide array of words and have practised using punctuation correctly. As they become more proficient in the mechanics of writing, they can confidently give expression to their original thoughts on the page.
At the age of 10 children would also undertake a regular dictation exercise to practice their spelling. (For more information on this see my post, How to use Copywork Cave for dictation exercises.)
*I highly recommend the Simply Charlotte Mason YouTube channel for more information