Windows: use the Character Map, or type Alt codes with the numeric keypad
Mac: press and hold the letter, then choose the accented version
iPhone/iPad: press and hold the letter, then select the accent
Android: press and hold the letter, then select the accent
Microsoft Word: use Insert > Symbol, or type the accent shortcut
Google Docs: use Insert > Special characters
HTML: use the appropriate entity code
Copy and paste the accented letter from a character table or online source