If you want to demolish a chimney, keep in mind it is a hard and dangerous task and requires assistance and proper safety gear.
Use scaffolding to have a flat, solid surface to work on. You will have to use a sledge hammer to brake the masonry down.
Use plastic sheeting to cover and protect the lawn below the chimney if any bricks fall off. Use a 2 weight sledgehammer to knock the bricks and masonry off. After the bricks at the roof line have been removed, reposition the scaffolding and work on the lower area. After you have removed all the pieces of the chimney, carry any debris away with a wheelbarrow