You can do it by two ways: The
E easier of the two is to type your document, and when reaching the point
where you want to underline, using a BT keyboard (sorry, don't know the
equivalent), type Enter with U. You are prompted about the state of the
underline, and if it is off, type N. This turns on the feature, and when
you have typed all the text you want underlined, repeat the enter-U and then
follow with F.
Alternatively, you can go to the first character and enter the Font
menu. Type U for underline, and follow the steps above. You then move your
cursor to the end of the underlining, and enter the Font Menu again, then U
for underline, and either f or n.
Hope that helps,
Sarah