Dig a pit about a foot deep and two feet wide. Make sure that it's far enough away from the shore that the tide won't flood it.
Fill the pit about 3/4 of the way with ocean water using your cup.
Place the cup securely inside the pit so that its opening is facing up toward the sky.
Take your sheet of plastic and lay over the pit. Make sure that you completely cover the hole you've dug. Be careful not to knock in too much dirt or sand while doing this.
Place stones and logs around the edges of the plastic sheet to hold it down. Use as many as you can. Make sure that the plastic sheet is stretched flat with only a little slack.
Seal off the edges of the sheet with sand or dirt. Pile it on as much as possible. This will not only prevent damage from wind. It will also help to keep animals out, and it ensures that the desalination process goes smoothly and efficiently.
Place a light stone or piece of wood in the middle of your plastic sheet. It must be heavy enough that it makes the sheet droop down in the middle, but it should not be heavy enough that the sheet actually touches the ocean water or the plastic cup. Also, make sure that it's not so heavy or sharp that it tears or rips the sheet.
Return to the pit after several hours, depending on the heat. The water should have condensed along the plastic sheet and collected in the cup, leaving the salt in the bottom of the pit.
