Fits: Shimano or SRAM freehub 

Size: 42mm x 35mm, 1.0mm deep

You will need appropriate tools to remove and refit the cassette, such as a chain whip, cassette lockring tool, and a large wrench.  Place the spacer onto the freehub first, followed by the cassette.  When tightening the lockring, please observe the manufacturer's recommended torque setting.  Check the cassette does not have any excessive movement (or 'play').