In the example from the documentation on Craft 3 'Video Embedder' plugin:
{% set embed = craft.videoEmbedder.getEmbedUrl('https://www.youtube.com/watch?v=6xWpo5Dn254') %}
{% if embed | length %}
<iframe src="{{ embed }}"></iframe>
{% endif %}
Can someone please give me full (i.e. total beginner!) instructions on how to get the URL to load.