Though I used TTL on my F90X here is what I did.
Match shutter (M) and F-stop to underexpose background -1 to -2, mostly the latter (TTL matrix) giving a shutterspeed around 1/8 sec, so the flash will give light to an
N-exposure of the main subject. Normally this will freeze movements within the flash range but not in the background, I like the effect and have seen only a few blurred pics. I use it alot when photographing family gatherings.