Set opacity anywhere from 10% to 90%, pick a font size, and rotate the text to any angle — the classic diagonal stamp is one option, not the only one.

Stamp the whole document at once, or restrict the watermark to a specific set of pages.

30-50% keeps the underlying text legible for a routine draft or confidentiality stamp; 70% and up is closer to what you'd use if you actually want to discourage someone from reusing the document.