In what? HTML, Word, or just writing. While typing german, we usually designated a umlaut by putting an "e" after the letter.
ae : ä / & auml ; (take out the spaces)
ee : ë / & euml ; (take out the spaces)
ie : ï / & iuml ; (take out the spaces)
oe : ö / & ouml ; (take out the spaces)
ue : ü / & uuml ; (take out the spaces)
Yes, I know there is only ae, oe, and ue, but I just included all of them. That is for HTML, by the way.
And just FYI, wrong forum by the way.