You can paste existing content from other documents or sources, but pasting from other sources (e.g., Microsoft Word, other websites) may bring unwanted "HTML markup" that could cause formatting issues. It may be preferable to paste your content as "plain text" (e.g., using Notepad or TextEdit), and then add the necessary formatting (e.g., bold or italics, headings, bullet points) using the formatting toolbar.
You can also try the "paste as plain text" button or the "Paste from Word" button if pasting from Microsoft Word. "Paste from Word" will retain your overall formatting from your Microsoft Word document, but will strip out any unwanted markup in the source. Additionally, the "Removing Formatting" button will allow you to highlight text and remove any unwanted formatting.