Use a worn cassette cog as a singlespeed cog

Singlespeed cogs are stamped, cheap, with deep valleys to avoid throwing chains. Cassette cogs are machined, expensive when new, have low profile teeth, and interesting ramps to aid shifting. Really nice singlespeed cogs are expensive, machined, with tall teeth.

Step 1:

USED cassette cogs can be free, but their teeth may be worn down, which skip, because the worn 'pulling' tooth edge is angled. Not good, right? This is called "shark toothing" by some people, and no, it's not good.

BUT! You can flip the cog around and mount it backwards, to use the unworn 'back' face of the tooth as the new 'pulling' face.