I couldn't find a really good guide to typing in extended characters in Linux (KDE specifically), so I've compiled what I've found into a table. Each OS does this a different way. I remember on Windows and OS/2 it was ALT + numeric id of character, so for the 'é' in résumé you'd have to remember that 'é' is 233 is extended ASCII. Or you'd just use the charmap application, which involved a lot of clicking. Mac OS X has it's own way of doing things as far as character combinations go, and there's always a little pull-down menu you can get to for obscure characters.
KDE is very nicely designed and has an elegant way of entering in extended characters. You type the compose key, which by default is the right 'Win' key. My laptop doesn't have a right 'Win' key, so I've configured caps lock as the compose key; I never use caps lock otherwise... it is an anachronism. Anyways, you hit & release the compose key, then press the keys which are shown in my chart. For 'é', it would be <compose> then apostrophe and e. The mnemonics seem to be easier to remember than Mac OS. I.e. e + ' = é.
Here is the table.
