Are you asking about an outer plastic disk on the crank outboard of the largest chainring or an inner plastic spoke protector disk on the cassette/freewheel (aka: "dork disk") between it and the rear wheel spokes?

Adding a guard to a crank is usually not easy unless the crank was designed for one or unless you replace the largest chainring with one.

Spoke protectors are available in several types depending on the largest cog size and the spoke count.