Start at the top. Remove the chimney above the roofline by basically tossing the bricks to the ground and cleaning up. Once below the roofline you’ll need to cover/repair the opening in the roof. In the attic you’ll need an opening or exit typically at a gable end to toss the masonry through. Get as low as you can. Go to the upper floor and remove the wall around the chimney. remove all masonry and tios them out a window. Get it blow floor level and patch the floor and finish out the exposed walls etc to regain the floor space.