I've used Pie Corbett's methods and think it works brilliantly. The premise is that "if they can't say it, don't expect them to write it".
You start off with the text genre you're learning about (story types, instructions, explanations, reports etc) and teach the children to 'tell' a basic text. You all learn to say it aloud by heart, adding actions to aid memory. You 'immerse' yourself in the text, and imitate it in different ways - draw maps to retell, role-play parts, do creative activities investigating settings, words and phrases, tell it in groups and pairs, 'box-it up', analysing the structure, sentences , grammar etc. This usually takes a few days, with not much writing, but loads and loads of talking.
Once the children are confident, you start 'innovating' - use the learnt text as a starting point, and start writing, but innovate and change the text. You might add more information, or adjectives, or more complex sentence structures, or rewrite changing the setting or main character, whatever your particular writing focus is. You then 'polish' your writing - editing, correcting, improving.
Last of all is 'independent invention' - you take what you've learned about a text genre and apply it in a different context to produce a piece of original writing. The whole process usually lasts 3 weeks.
Different schools will approach it in different ways, with different time scales, but essentially you 'talk' through everything and immerse, imitate, innovate and then invent.