Thread the retainer on all the way before trying to tighten the screws. Next tighten the screws evenly and firmly with your hand. If you need to tighten a little more use a screw driver or pliers to finish tightening. Take care not to tighten so much that the rubber washer squeezes out of the strainer under the sink.
Note: If the washer squeezes out no matter what you try just remove the washer and use extra putty on the top.