To begin, layer the cucumber and onion slices in a medium sized bowl or container. Set this aside for now.
In a mixing bowl, combine the vinegar, water, sugar, salt, and pepper. Give it a whisking.
Pour the mixture over the top of the vegetable slices. Cover with plastic wrap or the lid of your container and place it in your refrigerator for 3 - 4 hours. Occasionally give the mixture a stir while it chills.