u-tilde, again

OK, here's the relevant portion, hopefully in readable form. This applies,
at least, to Windows 98 and WP8.

1. Select Special Characters (Control W or click the icon) and get the
Phonetic or Multinational set--   Do this first, because if you do step 2
first, then you can't summon up Special Characters.
2. Click Format > Typesetting > Overstrike
3. Select and insert the desired diacritic, then (no space) type in the
desired letter (either order works), and click OK.

Contrary to what I said before, this IS explained in the WP Help file-- look
up "overstrike".  (My problem was that I kept looking for "strikeover".
Hmmph.)