How to put on social media links directing to my fb, etc?

I would like to put on social media links/buttons (directing to my youtube channel and my facebook page) in the footer or header of my catch box blog. I already tried your advice ("You can activate the social icons you want by adding in your social profile links through “Appearance => Customize => Social Links => Predefined Social Icons”, in your WordPress Dashboard) but nothing happened. What am I missing? Could you please tell me how can I add those links to the blog. Thank you. Best regards. Rafal