After reading the Quick Start Guide
In your HTML page add an element that CKEditor should replace:
<textarea name="content" id="editor"></textarea>
Load the classic editor build (here CDN location is used):
<script src="https://cdn.ckeditor.com/ckeditor5/10.0.1/classic/ckeditor.js"></script>
Call the ClassicEditor.create() method.
<script>
ClassicEditor
.create( document.querySelector( '#editor' ) )
.catch( error => {
console.error( error );
} );
</script>
Example
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>CKEditor 5 - Classic editor</title>
<script src="https://cdn.ckeditor.com/ckeditor5/10.0.1/classic/ckeditor.js"></script>
</head>
<body>
<h1>Classic editor</h1>
<textarea name="content" id="editor">
<p>This is some sample content.</p>
</textarea>
<script>
ClassicEditor
.create( document.querySelector( '#editor' ) )
.catch( error => {
console.error( error );
} );
</script>
</body>
</html>
This example is for the specific classic editor. FOr other variants, only CDN will change.