This is as easy as it sounds. Simply drop the washer into place then make all four nuts finger tight. Once they are all finger tight use a cross pattern to tighten and compress the strainer.
Torque each nut evenly and a little at a time but do not over tighten. It does not need to be cranked down just tight enough to compress the gaskets and make a water tight seal. Again torque the nuts evenly in a cross or star pattern similar to tightening lug nuts on a car.