Leading tone

Hi
What is the rules to write a leading tone chord, for 4 part harmony for strings?.

Do you mean a dim. vii chord or a V chord? Either way, remember that the leading tone itself can't be doubled if it's in the bass voice. Also, for the dim. vii chord (or for V7), the 4th scale degree should resolve downward to the third of the tonic chord. If you're doing a fully diminished vii seventh chord, have that 6th scale degree resolve downward to the 5th. I'm not sure if that would necessarily be the case for a half-diminished vii seventh chord. 

If you're going for a cadence to the tonic, I'd say to try your best to use contrary motion between the melody and bass line. 

I may be missing some things, as this is all from memory. 

Thank you