I was recently trying to send an email with the word résumé in it, but I couldn’t get my keyboard to input the characters I wanted without bringing up a character map. Here’s how you get your keyboard to print these characters for you.
Open System → Preferences → Keyboard (or run gnome-keyboard-properties) and click the Layouts tab, then click Options…. Under Compose key position, select an appropriate key. Right Alt works for me. Click Close then Close.
Now you can insert accented characters by holding Right-Alt and pressing a key, then a modifier, such as Right-Alt + u, then ‘ (single quote) to get ú or Right-Alt + n, then ~ (tilde) to get ñ.