There are two ways to answer his question.
First, the boys in the short story break down the house in a very systematic way as they plot mischief. One of the themes of the short story is the idea of destruction as a form of creation. So, T. and the rest of the gang dismantle Mr. Thomas' house. For example, the youngest member of the gang, Mike, cuts the electricity, and other boys work to destroy other parts of the house.
Second, the final destruction of the house comes, when the boys tie a rope to a foundational part of the house to the back of a truck. As the truck pulls away, the house comes crashing down.
A newcomer to the Wormsley Common gang, Trevor, or "T," as he is known to his mates, has some knowledge of architecture, since his father had been an architect until losing his position. T recognizes that Mr. Thomas's house, which has miraculously survived the Blitz, was designed by Christopher Wren. For reasons that are not made explicit, T organizes a wrecking crew, and over the course of two days, the boys dismantle the inside of the house using hammers, chisels, saws, and a sledgehammer. Directed by T, the boys tear down the plaster and panel walls, pry up the parquet floor, and cut the exposed wires. They saw through the stairway banisters of the spiral staircase, smash the bath fixtures, smash all the dishes, break all the ornaments, tear up the bedding, and tear up anything made of paper. T and Blackie burn a pile of money T discovers. The next day they saw through the joists, substantially weakening the house's structure. They weaken the mortar between the bricks of the house's inner walls with hammers and chisels and screwdrivers.
The coup de grace that brings down the house is engineered by a rope tied to a lorry in an adjacent car park. The driver unwittingly provides the force to reduce the house to a pile of rubble.