It's very simple. Stick the return label to the package you want to return.
Make sure everything is well packed so that it doesn’t get damaged.

Then take the package to a DHL collection point near you. You can find this return centre here:

Belgium: https://www.dhlparcel.be/nl/particulier/afhaalpunten of https://www.dhlparcel.be/fr/particuliers/points-denlevement

Netherlands: https://www.dhlparcel.nl/nl/particulier/servicepoint