<!-- required files -->
<script src="../assets/plugins/@ckeditor/ckeditor5-build-classic/build/ckeditor.js"></script>

<!-- html -->
<textarea id="editor1" name="editor1">
 ...
</textarea>

<!-- javascript -->
<script>
  var elm = document.querySelector('#editor1');
  ClassicEditor.create(elm).catch(error => {
    console.error(error);
  });
</script>