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Inside the shop, a feeling of warmth and welcome hit me and I was instantly nostalgic for the magical days of Christmas shopping in the school holidays, when a visit to Harrods, Selfridges, or Hamleys was as much of a treat as the illuminations suspended above Oxford Street and Regent Street.

Ah, the lights. The instructions about my assignment from the John Lewis press office were clear: "Wear weather-appropriate clothing and sensible shoes or boots. No skirts or dresses. Hard hat provided." Well, that seemed rather dull. This was supposed to be a "let's get ready for Christmas" column after all, so I decided to liven up my attire with a seasonal jumper featuring a fluffy breasted robin. Would Peter Cross, the store's communication director, mind that it came from M&S, I wondered?

I was next introduced to John Vasey, who runs Springfields, a company that designs and installs Christmas decoration and seasonal displays for leading high street retailers, shopping centres and airports all over the UK. He and his team have been organising the lights and decorations for the past 10 years at all JL branches. "We specialise in decorative things in high places," he told me. "The last 16 days were spent on the Harrods' lights, now we're doing Peter Jones and afterwards we'll be heading to Oxford Circus."

I was relieved to be dealing with the market leaders, as shortly they'd be responsible for my life as we ascended in the crane high above Sloane Square – but also slightly disappointed. At JL's Oxford Street store, John's team apparently abseil down the front of the building to attach the lights. Suddenly the cherry picker seemed something of a cop out. (Perhaps next year – Ed.)

_{(ANDREW CROWLEY)}

Disappointment gave way to fascination as John spelt out what exactly is required to create that shimmering curtain of white and silver light that adorns the iconic Peter Jones façade every Christmas, providing a gorgeous backdrop to the illuminated London plane trees in Sloane Square: around 9.5 kilometres (about 31,000ft) of lights and a whopping 45,000 bulbs. And yet this huge display uses little more electricity than it takes to boil a kettle – LED lights are apparently 90 per cent more energy efficient than normal bulbs.

I thought of the annual dilemma presented by decorating our tree at home: the desperate search to find a set of lights that work, the struggle with numerous knots that have mysteriously appeared over the year no matter how carefully they were packed away, and the stress of trying to fix the inevitable tripped bulbs. It was no surprise to hear that the Peter Jones LED bulbs operate on a bridge system so that if one is faulty, the rest will still work.

Then, the moment arrived. After a health and safety briefing I donned a hard-hat and harness and was carabinered onto the cherry picker with Ed Hickford at the controls. As we moved upwards I had a taste of what it must feel like to be Justin Bieber or Kanye West, who've both used cherry pickers to surf their screaming fans at concerts (rather more glamorous than departmental light installation).

_{(ANDREW CROWLEY)}

We began working at the giddy heights of the seventh floor. There was room for about six reels of lights in the cherry picker (along with the me and the crew), and we screwed one end of each reel to fixtures on the building. Ed then lowered us down slowly to canopy level allowing the reels to unravel – no knots, I noticed – and drape beautifully down the front of the building. The line of bulbs was then secured at various points to avoid flapping and tangling. We repeated the process until the whole store was covered. It was slickly done and once I had a feel for it I asked Ed if I could have a go at the controls. He was surprisingly hesitant. Perhaps he had a vision of the tabloid headlines – "Sloane Danger: Pippa Pops In To Peter Jones – Through Third Floor Window" – should I lose control. But I eventually managed to persuade him and for a few empowering minutes I was Bob the Builder in my yellow machine, busy helping to put the stardust into Christmas and to transform the drab and grey of our winter into something rather wonderful.

So the windows are done and the crowds are busy snapping away at them with their iPhones. The in-store decorations are up and now the outside lights are twinkling brightly. And yet there is one ingredient missing from the John Lewis Christmas, something that has become a staple of the season. Who can forget the lovestruck snowman of last year? Or the little boy from 2011, who counted down the days to Christmas when he could give his parents their presents?

Tonight, the 2013 advertisement has its TV premier and there's a growing buzz about it. We know, from teasers on social media, that it features Lily Allen singing Keane's "Somewhere Only We Know". But what else? I know where I'll be at 8:15pm.

Pippa's picks: five things to try

1 Cherry: A Life of Apsley Cherry-Garrard by Sara Wheeler: a fascinating biography.

2 Festive knit: I love Joules's Intarsia fox and partridge designs.

3 Best advert songs: Sniffy Dog's Whispers and Stories (VW Polo) and Welcome Home by Radical Face (Nikon Camera).

4 Pucket: a crucial game for every family Christmas. Beware, it gets aggressive.

5 I love LED: fairy lights add early winter twinkle to everything.

Pippa Middleton's morello cherry Bakewell tart recipe