Avocados brown quickly, so I suggest only cutting what you need for that layer.
First, cut one of your limes and grapefruits into quarters. Next, wash and cut your watercress to your liking. I usually pull the leaves and leave some of the stems. Put the watercress at the bottom of your serving dish. Next, cut your avocado into chunks and layer on top of your watercress. Go ahead and squeeze the lime juice from one of the quarters on the avocado layer to keep it from browning followed by lightly salting. You'll want to chop the onion into chunks and quarter the strawberries and layer these on the avocado. Next, tear pieces of cilantro and sprinkle on the onion and strawberry layer. The last thing you'll do is squeezing on of the quarters of grapefruit juice on that layer.
You'll repeat these steps until you have run out of ingredients. After completeting your layer, juice the lemon, lime, and grapefruit on the very top layer and add salt and pepper to taste.
You can now chill this or serve it straight to table!