Even professional writers cannot always tell you how they do it. Press them on it and they tend to talk in metaphors. EM Forster said writing is like dipping a bucket into the subconscious and bringing up parts of yourself that are normally beyond reach.

I have been dipping that bucket since I was five. I did not make any money from it until I was 40 when my first book was published, but money was never the motivation. I was more interested in the contents of the bucket and I still am.

That is not to say writing has not had other uses. When as a penniless student I fell in love with a girl at university and realised on her birthday that I couldn’t afford to buy her a present, I wrote her a fairy story instead, half-expecting to be shown the door. To my amazement, she liked the story and the following year asked me for another one. She is now my wife, we have been together for 25 years and she has had 25 birthday stories; and somehow I have produced six novels. We both became teachers but whereas she is now a head of English, I eventually left teaching to write full-time.

Why is writing so tricky? Because it requires mastery of two conflicting skills: a creative skill and a critical skill. The former is of the imagination, the latter of the intellect, and they come from different brain hemispheres. To write well, we have to employ both to maximum effect.

For me, the best way to do this is to give each its turn but not to try to do the two together. If we try to be creative and critical at the same time, we run the risk of stemming the flow of inspiration or having one skill dominate the other, leading either to stories that are well-structured but lacking in invention, or stories that are rich in invention yet lack shape. By giving the imagination free rein first, as in brainstorming, then bringing in the critical hammer afterwards, both skills operate without constraint.

It is not the only way to write, of course, and many writers do indeed manage to create freely and yet edit themselves critically as they go along, but it is the way I write and I usually encourage pupils in workshops to experiment and see if it works for them, too.

It often does but they can be timid at first and I sometimes find their imaginations have been cramped by a tendency to over-plan. There is nothing wrong with planning in itself. Some writers plan in detail before they start writing, but many do not, and some do no planning at all. I do very, very little. Every story I have ever written has changed so many times during the course of the writing and ended up so unlike my original conception of it, that a predetermined plot would have been pointless. What I have learned instead is to trust my imagination to guide me, to accept that it will occasionally lead me down wrong avenues but also that - given time and patience - it will help me find the true north of the story.

It does not just work for fiction. This article has been written in exactly the same way. Children should be encouraged to trust their imaginations. Some will need structure, to be sure - guidelines, modelling, spider diagrams, whatever is helpful - but some will need no more than a few hooks to start the story off: a boy carrying a baby, a country lane at night, an old woman watching in secret.

Writing, for all its challenges, is one of the most enriching activities children can undertake. Stories and poems should be given as much weight as more formal writing exercises. We should encourage them to write not just for grades and exams and teacher approval but for themselves. We should tell them that it is good to dream, that the stories and poems they have in their heads are as unique and special as they are, that it is OK to write bad stuff in the process of producing good stuff, that fear of failure is normal and is something we all have to deal with. We should tell them that writing is precious, that it is a tool for communicating, not just with others, but with ourselves; that it is a mystery worth exploring.

Tim Bowler’s new novel, “Starseeker”, will be published in September by Oxford University Press. His book River Boy won the Carnegie Medal