how do i fit this sprocket in this shaft of the magnet?

i'm aware there's a nut a washer
http://www.electricscooterparts.com/SPR-2511.htm

but i don't know exactly how to secure it.

That motor shaft was meant to slip fit into a planetary gear box, if there is enough shaft left after the sprocket is on the shaft (with motor running) you could use a mototool type grinder with a cutoff wheel to grind a retaining ring groove into the "d" shaft