Measure the attic to determine how many reflective tiles you will need.
Remove any floorboards to give complete access to the attic joists. Some may have to be unscrewed, but commonly they are placed on top of the floor boards.
Place the tiles in the gaps between the ceiling joists. If one needs to be cut use the garden shears to cut the material. It will give a cleaner cut than any other cutting device.
Replace all of the attic flooring. You can screw it down if you wish, but it is not necessary.