The first of these I learned from a stern chief sub who had received a sort of military training on a provincial newspaper (she carried her own spike from job to job); the second from a limp journalism student who did work experience one day in my office.

“What do they teach you at journalism school?” I asked this girl. “What do they say about interviewing people, for instance?” She bit her lip. “Well, remember to carry your bus fare, and take a sandwich.” “I see,” I said, trying not to sound too disappointed. “So did you remember to bring a sandwich today?” “No,” she said, and burst into tears.

Unlike common sense, however, journalism can be taught. It is a craft, principally. Not starting with a quote is a good idea not only for stylistic reasons, but because the fashion for drop-caps (big initial letters at the start of the piece) does not accommodate inverted commas. The trouble with journalism - from the Masterclass point of view - is that you can’t practise it at home, as you might write a play or short story. Journalism only exists because someone wants to print it, and is offering money - which certainly makes it more exciting. If you send unsolicited material to a newspaper (“Here is a court report I have written”), hacks will not be impressed by your enthusiasm; they will regard you as weird and desperate, and put your copy in the bin.

The most fundamental fact of journalism, however, is that it is not one job but two, with two distinct functions - writing and sub-editing - and that it is better for a writer to embrace the hard fact of sub-editing at the outset than to wish for ever that it didn’t exist. Both roles are concerned with the battle to transmit words in the best possible form, yet oddly the armies are not on the same side. To the sub-editor, you see, written words are not sacrosanct. They are wild raw material to be roped, broken, and fitted exactly into columns; they emanate from vague whingeing buffoons who don’t check spellings, can’t work the photocopier, and have no notion of syntax.

The characteristic sound of a sub-editor is the sucked tooth, followed by a tut-tut and a high-pitched cackle of glee. To the writer, however, words are rather different: they are precious babes, borne of hard labour, presented tremulously (“Be gentle!”), and then skewered alive for sport. In the mind of the writer, the average subs’ desk is the equivalent of the King Herod Pre-School Fun Club. The sub-editor is the Superego; while the writer is the Id.

Writing for a newspaper may look like a nice way of talking to people without having to offer them a drink, but when a sub-editor removes your first punchline, think again. Sub-editors change words, move jokes, cut sentences in the middle. And the irony is, they assume they are making improvements. I know this, because I was a sub-editor for 13 years and blithely rewrote some of the biggest brains in England. Now I am in the enemy camp, I feel quite sick at the recollection of former crimes. A friend once wrote “metaphysical” in a review for The Times Literary Supplement, and found on publication it had been altered to “strange”. It was a shock from which his literary sensibilities never totally recovered.

The aim of all journalism is clarity. Brevity is also prized; elegance is occasionally permitted. Jokes are definitely extra, and are therefore to be found littering the floor of many an editorial office, making faint “ha ha” and “please help me” noises, as they are trampled underfoot. Cutting is therefore the main skill involved both in writing and editing: leaving out is as important as putting in. Readers of newsprint haven’t time to absorb much nuance; if the punctuation makes them stumble, they’ll give up. When I first wrote for The Times in 1985, I saw a woman on the Putney-to-Waterloo train open the paper at the right place, smooth the fold, and settle down to read my piece. Half-way through it, she laid the paper aside, and looked out of the window. I will never forget my mixed feelings. “It gets better, lady!” I wanted to assure her, but I had failed to arrest her attention beyond Clapham Junction, and that was the truth.

Professionalism counts for a lot in this business. It counts for everything, in fact. If someone asks you to write 400 words, you must write 400 words. If the deadline is Tuesday, meet the deadline. “I did 1,000 because I felt it was more, you know, appropriate” is a sentiment that will earn you nothing except heartbreak, because the 350 words that finally appear on the page will be the ones you liked least. Lateness with copy is simply unforgiveable; writer’s block is not an option. “Don’t get it right, get it written” they tell you, just to rub it in. Rebecca West said journalism was only “the ability to meet the challenge of filling space”, and that’s much less cynical than it sounds. Filling space is no small challenge, especially under pressure. Columnists on newspapers sometimes lose their nerve, you know, like Spitfire pilots. They take secret breaks in health farms, where they wake up screaming and bat the air with their arms.

My own journalism is hardly cutting edge, incidentally. I am at the comfy, softly padded end of the spectrum - opinion, columns, profiles, TV criticism. A first-hand report of colonic irrigation in Knightsbridge is the toughest assignment I have ever faced. In this upholstered corner of the business there is more freedom for style and humour, for finding a personal voice. But I have struggled for years to get it. The jokes I used to insert in book reviews for The Independent on Sunday were wordlessly removed as lapses of propriety - week after weary week - but I still put them in, hoping that one day a sub-editor would be called to the phone at the crucial moment of excision. It worked. Forging a personal voice in journalism, you discover how those weeds must feel that have pushed through cracks in the pavement.

And alas, getting into the profession in the first place is even harder without supreme luck. Having a famous literary or political parent is of course a bonus; nepotism sits as comfortably in what used to be called Fleet Street as it did in Renaissance Italy. If your parentage is ordinary, however, you must either change your name to Sackville-West or curb your natural annoyance. I remember complaining to a Fleet Street editor about jobs-for-the-kids, and he pointed out in all seriousness that miners’ sons got work down the mines (in the old days). So it’s all fair, apparently. It evens out.

Otherwise, a degree in media studies is not the right thing; it gives you theory and high-mindedness, but not the three-finger typing. A vocational course is the answer - preferably one which credits you with sufficient intelligence to pack your own lunch. The City University and the London College of Printing are famous ones. Simon Hoggart of The Guardian recently sampled a journalism correspondence course anonymously, to test the standard of the tuition, and was alarmed by the result. Having started a homework report in the standard newspaper fashion, “Happy faces at Hove Primary School today whenI”, he was marked down for subjectivity. A Gradgrindian force is clearly at work out there, so beware.

So it’s back to the sandwich, I’m afraid. It seems to be the best tip I’ve come up with. Writing journalism, the trouble is that nobody will tell you how it’s done; they simply rewrite you to fit the bill. When I handed in my first ever piece for publication, I expected a mark, lots of pointers in red ink, and a day’s grace to revise. Instead it was accepted, cut to fit, and printed. I experienced that day a kind of vertigo which has never since quite left me. Editors do not tell you what they want: “Just write it,” they say, “and we’ll decide later.”

An insecure person can spend many morbid hours poring over the mangled body of a once-lovely piece, trying to reconstruct the motive behind each wound, bruise and amputation, but it’s a complete waste of time. Remember the bus fare. Remember the sandwich. And good luck.

Lynne Truss is principal TV critic of The Times and “Columnist of the Year” for Woman’s Journal. Her novel Tennyson’s Gift is published by Hamish Hamilton, Pounds 16.

Next week: Andrew Davies on adaptations