For adding one-line instructions or clarifications, try to use Help Text and Hint Text.
To add instructional paragraph(s), use the 'Text' field:
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.