"I thought my wife had gone mad when she gave me an empty pizza box for Christmas," Graham Shields tells me. "I opened the wrapping and there it was. An empty pizza box." Then he saw the white slip of paper inside with the details of the pizza-oven building course his wife had bought him. "I realised what she was basically saying was, 'You go and learn how to do it and then come back and build us one in the garden.'"

He's not alone. Of the 12 of us on the course, most say they were strongly encouraged by their families to attend. A pizza oven, it appears, is this year's must-have. "You could buy one from Jamie Oliver at three grand a pop," the course leader tells us, "or you can build one for nothing using materials from your garden."

The day's lesson is led by Kate Edwards, who runs them in her large backyard in rural Norfolk. She starts by teaching us how to identify the materials we need. "This is the most important thing I'm going to teach you. All the rest is easy as long as you recognise the materials."

All 12 of us gather round to watch Edwards dig a hole.

"Can you hear that noise?" she asks. "That crunching sound? When I was digging through top soil there was no real noise – it's kind of soft."

We take it in turns to peer down the hole she has dug and, sure enough, the chocolate-brown topsoil has given way to a seam of crunchy gravel.

"We dug over the whole of our garden and it was incredible how different the soil was – clay here, beach sand there," she says. "Go home and dig a hole and see what comes out of it."

To make cob, we need 25% clay, 75% sandy gravel. Luckily Edwards doesn't make us pepper her garden with holes, but instead gives us buckets of clay and sand she herself dug out the night before. Then she orders us to "dance" on the mixture. Basically, this involves throwing the clay and sand onto a tarpaulin and jumping up and down on it, mashing it up with out boots until it becomes a smooth, even paste. That done, we divide the batch. Keeping one quarter back, we add half a bucket of straw to the remaining three and mix it in.

We leave the cob to one side while Edwards shows us the base she prepared for us earlier. It's a round brick structure that looks a bit like a wishing well. She's filled it with rubble and topped that with sand to make it smooth and flat – ready for us to add a layer of firebricks. This will be the cooking surface. After that, we build a dome of wet sand to act as a "former" – the support for us to build the cob around.

Next to me a man called David groans audibly. "It's so obvious when you someone tells you," he says. "I've already built the base but I just couldn't work out how to make the dome." We get talking. He muses over whether he might be able to use the oven to smoke fish, but in the end we decide not – adding a chimney would let out too much heat and the pizzas would never cook. He tells me about his friend's Bramley orchard and cider press – he makes 120 gallons a year, "enough for family and friends" – and of the still he is hoping to procure in order to turn some of the cider into calvados.

By now the sand dome is perfect, ready for David and I to help the others lay two skins of cob over the sand. The inner skin is cob without straw – it will be in contact with the flames, so any straw would catch light. The outer skin contains straw, which gives the cob strength, acts as insulation and helps the dome to dry out quicker.

And that's it. We cut a hole for the door, pull the sand out, and the oven is built. It's unbelievably quick. Edwards says the whole thing – the base as well – can easily be put together in a day. Although, for environmental reasons, she strongly recommends building the brick base with limecrete rather than concrete – and while concrete will dry in a couple of hours, limecrete will take a couple of weeks. Also there is the small matter of digging up enough clay and sand, which thankfully we didn't have to do.

I say thankfully not because I was keen to avoid hard labour, but because it did leave us quite a lot of time for testing the pizzas. They were terrific.

• Frederika Whitehead attended Kate Edwards's one-day cob-oven course near Acle, Norfolk, priced £80 including lunch.