Use the subscript button in your editor, usually marked with x₂ or a similar icon
In Microsoft Word, select the text and press Ctrl+= on Windows or Command+= on Mac
In Google Docs, select the text and use Format > Text > Subscript
In HTML, wrap the text with `…`
In LaTeX, use `_{…}` for subscript in math mode
In Markdown, use HTML tags like `…` if supported
In plain text, use Unicode subscript characters when available
In many apps, right-click the selected text and look for a subscript option