I don't want to sound like a broken record, but...once again, Charlotte Mason's method for this subject is simple, effective and natural. Charlotte's students learned composition by continually reading living books and narrating what they had read.
Throughout history, one of the most effective and most common ways people acquired skills was to apprentice. In an apprentice relationship, the apprentice works alongside the master craftsman, assisting him in his work, all the while learning the life skills necessary to one day continue on his own as a master craftsman himself. The apprentice does real work while he gains an understanding of the art and practical skills his master shares with him.
Narration, the simple retelling of what a child has read or heard read, is composition. In effect, the children are the apprentices of master craftsmen like Charles Dickens, Thornton Burgess, Beatrix Potter, and C. S. Lewis. The children study directly under the authors (by reading their works), learning their style, paying close attention to the way each author weaves a story together, the vocabulary they use, the structure of their sentences. They learn what sound right and what doesn't. They learn when to use colourful descriptions and how to make use of quotations. The best part is they learn it all simply by performing the most enjoyable of tasks -- retelling the story they have read.
In Charlotte's schools, the children narrated orally beginning at age 6. Continuing with oral narration, written narration was added at about age 10. Formal and specific teaching in 'essay-style' writing was not added until age 14 and was fairly minimal. Charlotte found that when children were experienced in oral and written composition through narration, the skills of learning essay writing were very simple. I have also found this to be true. With my 14 year old daughter this year, the teaching of essay writing was essentially a one-afternoon task. It was so simple because she had long-ago learned the art of effective composition and storytelling. Describing and implementing the essay format was very basic and she picked it up effortlessly.
A word about beginning written composition/narration: for the first couple of years, I encourage you to allow your child to just write without giving them any rules to follow. Let the flow of their writing develop without their feeling hampered by trying to remember to vary the length of their sentences, to use quotation marks in just the right spots, etc. I like to help our children make a 'good copy' of their written narrations later on. This is eased by the use of the computer, which makes correcting a simple task. In our family, I have our children do a combination of handwritten and typed narrations.
We are conditioned to think that to truly learn something, it must in the boring format of: memorizing a formula with lots of rules and applying them in workbook/test format. Anything else seems almost a bit of a gamble. In reality, though, how many of us would be operated on by a surgeon who had never worked directly under another surgeon? What about having your house wired by an electrician who had never actually seen anyone do proper wiring? The real gamble, then, is to assume our children can ever learn to write effectively without studying under master writers.
Karen Andreola, in The Charlotte Mason Companion, says that Charlotte's method of learning composition is so simple that it feels as if we are cheating. That is music to this hard-working, homeschooing mom's ears.