This one is pretty simple. You each have one of the insoluble chlorides. (Review your solubility rules).

So remember rinse all glassware with distilled water. If you have stray Cl- ions from city water, it could be a problem.

Testing for complete precipitation.

Precipitation must always be complete. That mean you have separated the insoluble chemicals from the soluble. You can't leave the insolubles behind due to incomplete precipitation. If you do you'll probably get a false positive on the next procedure. It assumes you have stripped your unknown clean of everything that comes before that step.

Add one more drop of reagent. If the liquid stays clear, you have complete precipitation, if not add 2 more drops of reagent and centrifuge. Then test again with 1 drop of reagent.