Use a potato peeler to peel the cucumber if you don’t like cucumber skin.
Cut the cucumber into thirds, half the thirds lengthways and scoop out the seeds with a teaspoon.
If you are using large tomatoes, half them and remove the seeds and any green stalk.
Cut all the tomato and cucumber into small equal sized pieces. No need for a tape measure, but 5mm pieces will give you a nice mouth mix.
You want the same quantity of prepared cucumber and tomato and onion when prepared.