It is JK Rowling’s favourite room on Earth: the place where the words she wrote come alive in a way that can seem as magical as Harry Potter’s wizard-filled world. The props from every single film, from Quidditch World Cup posters to the Marauder’s Map, were created in this special place in Hertfordshire.

It is here – at Leavesden studios in the Warner Bros complex – that the design duo Eduardo Lima and Miraphora Mina (whose own name sounds like it could have come from the pen of Rowling) had to “think and breathe Harry Potter every day” from 2000 and 2010.

“We were thrilled when JK Rowling first showed up at our studio,” says Mina. “She loves it: she has called it ‘the place where objects come to life’ – and she has even taken some of our props home with her.”

“We are proud parents of every prop,” says Lima. “Definitely” agrees Mina, as she points to an alphabet poster on the wall that reads: A is for Alchemy, C is for Cauldron and W is for Wizard. “This hung on Harry’s bedroom wall at Godric’s Hollow. Like Dumbledore’s Memory Vials, it is one of the ‘lesser seen’ objects from the Harry Potter films that are finally getting some limelight.”

Harry’s hand-drawn childhood poster is one of many prints and props on show at a new MinaLima exhibition in London, The Graphic Art of Harry Potter Films. Bursting with attention to detail, it is a rare opportunity to “properly view things that are only momentarily seen on screen”.

The London-based duo have done graphic design for many films – from The Imitation Game to The Fault in Our Stars. As movie prop designers, explains Mina, “anything that needs to convey a message visually comes through us – from street signs to newspapers, potion packaging, sweet wrappers, books and any piece of paper with writing on it. And we give it all the same level of attention.”

The Daily Prophet might just be “the single most labour-intensive prop ever”. A single edition of the wizarding tabloid took them two weeks to make – and they sent more than 30 editions to print altogether. “The newspaper’s tone changed over time,” says Mina. “In the Prisoner of Azkaban it gets darker and more serious. Then in the Order of the Phoenix, when the Ministry of Magic takes over, it transforms into propaganda.”

“There is one sequence where the camera actually travels inside the paper – so we had to write lots of funny adverts from scratch so it looked realistic.”

The items are arranged chronologically, starting with Harry’s Hogwarts acceptance letter (famously addressed to the Cupboard Under the Stairs) and ending with the Undesirable No 1 poster. Visitors can read the Mudbloods pamphlet, examine the small print in a Ministry of Magic public notice or revel in wonderfully colourful Quidditch World Cup posters.

There is a decorated, platform 9 ¾ London-to-Hogwarts train ticket, some Chocolate Frog packaging, the book Fantastic Beasts and Where to Find Them by Newt Salamander and Educational Decree Proclamations (decree number 31 states “boys and girls are not to be closer than eight inches from each other”).

And there is plenty of mischief, from Dung Bombs to the Nose Biting Tea Cup. “When we made the Weasley’s Wizard Wheezes joke shop, we had to approach it like teenage boys with zero design sensibilities,” jokes Mina. And with their clashing graphics, disconcerting colours and chaotic mismatched type, they do look like they were clumsily put together by a child.

Their designs manage to look both ancient and current at the same time. “Even though the stories are set in the present day, we chose to take historical visual styles and shift them slightly into our fantasy environment, rather than invent a new language from scratch,” says Mina. “It’s a magical world that is anchored in real life.”

MinaLima’s “passion for typography” is clear, especially in classic pieces like The Marauder’s Map (Mina’s favourite). “We knew it had to be cunning, intricate, unconventional and witty,” she says. “We wanted to show the complexity of the school’s architecture, that it is made up of layers and corridors and is quite unfathomable!” She found some beautiful 18th-century animal drawings, then traced them over the actual architectural drawings of the Hogwarts film sets.

“I drew it all by hand in pieces that were then assembled digitally and printed on a humble photocopier,” says Mina. “Nescafe Gold Blend is our secret ingredient for the all-important final ageing ... but we can’t reveal our exact alchemical formula!”

As well as the movies, MinaLima have spent years designing the Wizarding World of Harry Potter at Universal (in Orlando and Japan). They attend Harry Potter conventions all over the world where they are “moved to tears” by fans they have inspired. “Some tell us they are thinking of doing graphic design because of us,” says Mina. “And some come up and hug us and tell us we changed their childhood,” says Lima.

“I come from a small village in Brazil,” says Lima. “I still can’t believe I am working on the biggest franchise in the world.”

• The Graphic Art of Harry Potter Films is at Coningsby gallery, London, until 19 December.