TRACON Controller providing OJT described a loss of separation event during attempts to vector aircraft below an overcast layer for visual approaches; the reporter acknowledging a failure to intervene in a timely manner.

Narrative

I was providing instruction on the north final to Runway 26R. We were doing visual approaches; but had to get the aircraft down and close due to clouds and visibility. The Developmental was working a situation closer to the airport with a few airplanes trying to keep them separated. The CRJ2 came over slightly high and fast; so with the cloud base at about 6;300 MSL; we had to get the aircraft down to get the airport or proceeding aircraft insight. The B717 was inbound to Runway 27L; and had not seen the airport yet and was turning into final at 7;000 FT. Timely action was not taken to put the CRJ2 into a place to do anything with him; so my Developmental was going to try and turn the aircraft toward the airport; but I instructed him to turn him to the left to pass behind the B717. Since I was providing this information; it was not done in a timely manner; and the two aircraft loss separation. Make sure that my Developmental had a plan to work the aircraft into a position and altitude that would work him into the sequence in a safe manner. I should have controlled first; and instructed after as to the reason for my actions.