I first learnt how to make a map at the age of about eight, when our teacher showed us how to pace out the length and breadth of the school playground and draw it on paper. Such power! To see things as if from above, and to make different sorts of marks on the paper to show trees, and rivers, and walls and buildings… I went home that day and immediately started on a map of the house and garden. It must have been about then, or only a little later, that I read Treasure Island for the first time, and realised that you could make maps of places that didn’t actually exist, and show where treasure was buried. It was intoxicating.

I begged for an atlas for Christmas, and was given the smallest one available, which at least had the merit of being pocketable, so I could carry it about. I’ve still got it. I brooded over that atlas for hours, planning journeys of exploration to the emptiest places I could find, and gradually came to discover that the physical maps were in fact more interesting than the political ones, because they showed mountains and deserts and the depth of the oceans and so on. I marvelled at the immense size of Greenland: only much later did I grasp that that immensity was an effect of Mercator’s projection. I had no idea what Mercator’s projection was, though; for a long time I just liked the sound of the words.

The first few things that I wrote (apart from the inevitable teenage poems) were set in the real world, or the one that we fondly imagine to be real. I didn’t need to make new maps, because there were plenty of maps of London already, and I had a stack of them on the desk or pinned to the wall as I wrote: reproductions of old Ordnance Survey ones in particular, which showed every single building in Clerkenwell or Stepney, and where the docks were, and the shortest route from Limehouse to Bloomsbury.

They were invaluable, precisely because they were real. If you stood on this corner and looked that way you could see St Botolph’s Church. In Wapping there was a sinister place called the Animal Charcoal Works: surely I could set a scene there. If the year happened to be 1878, and you happened to be strolling along the new Embankment, you might see Cleopatra’s Needle, just arrived from Egypt, in the course of being erected. In the interests of plausibility, I was a meticulous realist.

But when it came to the map of an imaginary place like Razkavia, for which there were no maps available, not even in the great geographical emporium Stanfords of Covent Garden, there was nothing to do but make one up. So I happily sat down with my pens and coloured pencils and set to work. I wanted it to show the city in detail, with the University, the Botanical Gardens, the Rock of Eschtenburg, the funicular railway, the Café Florestan, all that sort of thing; and the old crooked streets; and I wanted to show where it was in relation to (as I say) Bohemia, Prussia and so on; and I wanted to show a few other features of Razkavia, such as the Falkenstein Mountains, where they mined nickel (of great interest to the military authorities of the neighbouring great powers), and the fashionable spa town of Andersbad, where they’d paid Johann Strauss to compose and perform an Andersbad Waltz, which was unfortunately not one of his best, and the wild forest of the Ritterwald. And so on…

I had a vast amount of fun drawing this map. Drawing is infinitely preferable, as an occupation, to writing. Writing is a matter of sullen toil. Drawing is pure joy. Drawing a map to go with a story is messing around, with the added fun of colouring in. Unfortunately the map I drew didn’t pass muster: the publisher raised an elegant eyebrow, pursed her lips and tapped her fingers on the fine 18th century desk before pushing my piece of paper away with a silver pencil. “Yes, well, we’ll have to get this done properly,” she said. So it was.

Sandi Toksvig: ‘Look up from your mobile phone – you don’t need to go far to see great marvels’

One of my favourite maps – although, sadly, only copies now exist – was made in Ebstorf in northern Germany, sometime in the 13th century. It’s massive: 11ft 6in (3.5 m) square, a giant bedspread of a thing. The largest of all the medieval charts, it was made from 30 pieces of goat skin sewn together. A mappa mundi, a glorious piece of idealised art attempting to record what was then known of and in the world. It was found, by chance, in 1830 in a convent storeroom, and it’s a marvellous visual encyclopedia, rich in detail and beauty – a map of lands and stories.