Soak the lentils for 1 hour in cold water. It's not absolutely necessary but it does help to digest them more easily.
Get rid of the soaking water and cook them from cold water over medium-low heat. Let it simmer gently to avoid them going hard. From the moment the water boils, it will take 15-20 minutes to get them al dente. Don't cook them more than that or they will disintegrate in the salad. Remove the lentils from the water and let them cool down completely.
Cut the cucumber in half lengthwise. Use a spoon to scrape out the seeds. Cut it in cubes.
Some kale leaves have a thick stalk; it's best to remove it because it's not very nice to chew on. To remove it, fold the leave in half leaving the stalk to one side. Use a knife to cut it off. Then chop slightly the leaf. For leaves with thin stalks, you can just cut them up.