For adding one-line instructions or clarifications, try to use Help Text and Hint Text.

Plain Text Instructions:

To add instructional paragraph(s), use the 'Text' field:

1. In the form editor, click 'Add Field'.
2. From the field type selector, pick 'Text'. It'll be in the Informative Fields section.
3. Type your instructions against the 'Content' area and click 'Done'.
   

Rich Text Instructions:

If you'd like to your instructions to be formatted with certain words made bold, italics etc., please use the '*Markdown*' field. Markdown offers an easy-to-use way of formatting rich text. To know more about Markdown, please see Mastering Markdown.

If you are comfortable with writing HTML, you can do that too. Just use the 'HTML' field.

Adding Images & Videos:

You may also add images and Youtube videos to your forms. Just use the respective field types.