xaltfoxy (In Windows) You can use <ALT>0235 to get ë. You just need to know the value to use - I get them from the 'Insert Symbol' option on LibreOffice, or you should be able to copy/paste them from elsewhere.
raoul On Linux, you need to enable your Compose key in your keyboard settings (I have caps lock set to be my Compose key). Once you've done that, it's as simple as:<Compose>+<Shift>+", e => ë<Compose>+', e => é