In a word, the boys simply decide to destroy the house from the inside. T. envisions this destruction happening from the inside out, being completely imperceptible to the outside world and being something that the gang can accomplish:
We'd do it from the inside. I've found a way in... We'd be like works, don't you see, in an apple. When we came out again there'd be nothing there, no staircase, no panels, nothing but just walls, and then we'd make the walls fall down- somehow.
Once the boys vote on T.'s plan of destruction, he is able to disseminate orders and instructions as to how exactly the house will be brought down. He ordered them to "bring some big nails" and a "hammer and a screwdriver," as they will need "plenty of them." He also asks them to bring a saw and some chisels. The intent is for them to focus on the work of tearing down a house from the inside with brute force so that it too becomes a gutted remain like so much around them.
he destroyed the house in different stages for e.g. the door frame the banisters the panels and chisels etc and flooded it in the end. they destroyed it first from in and than from out at night no that no authority will chase them.