When a child has been able to trace and draw lines with the metal insets and has good pencil control, writing is introduced while learning the sandpaper letters.
Take a sandpaper letter that your child knows well with an unlined piece of paper and pencil. Make sure your child is seated at a table that is the correct height for writing. Your child feels the letter a few times. Afterwards, you write the letter on the paper in the correct form. I write a larger letter than normal because younger children usually write bigger letters than adults. Give the child the pencil and show how to feel the letter again. Let the child write the letter, feel the letter again and write the letter a second time. Many times children will repeat this until they have mastered writing the letter.
It is much easier to learn how to write correctly from the beginning. Montessori teaches cursive writing and not manuscript. This way a child can master one way of writing without having to change writing styles.