Use HTML: `text`
Use Markdown if supported: `H~2~O`
Use LaTeX: `H_{2}O`
In Microsoft Word: select the text and press `Ctrl` + `=`
In Google Docs: select the text and use `Format` > `Text` > `Subscript`
On Mac: select the text and press `Command` + `=`
In plain text editors, use a Unicode subscript character if available
In programming or markup, use the subscript syntax supported by that language or platform