I know what I wrote sounds sort of confusing. The reverse adapter has a k-mount on one side (just like a regular lens) and threads on the other side (just like a standard filter).
You screw the adapter onto the front of the lens where a filter would go, then mount the k-mount side of the adapter onto the camera. The normal k-mount of the lens will then be sticking out toward your subject.
Btw... Focus is achieved by moving the camera fore and aft since the focus ring will have no effect in this configuration.
You can find the filter sizes for many Pentax normal primes at
Pentax Normal Prime Lenses at
Bojidar Dimitrov's Pentax K-Mount Page (an invaluable resource).