As for making the trill appear correctly on the page, it's rather fiddly.
There is also more than one way to skin this particular cat. Here's mine.

The first thing to make clear is that a trill instructs the player to
rapidly alternate between the written note and the one which is above it in
the scale. So, in C-major, a trill on C indicates C and D; and a trill on E
indicates E and F. I.E. the first trill is a whole-tone and the second is a
semi-tone.

In fact, there are some people who say that a trill always indicates a
semi-tone trill unless stated otherwise, though this is a more modern
interpretation. In Classical and Baroque times, it was often left up to the
player to use common sense. What this all boils down to is that it is best
to specify both the upper and lower note.

Specifying the lower note is easy because the lower note is the one written
in the score. I.E. if you want the lower note to be an F-sharp, make sure
that you create an F-sharp on the staff.

The upper note is more difficult because you need to put an accidental above
the trill to indicate what the flavour of the upper note is. Remember that
the letter of the note will always be the one immediately above the letter
of the written note.

The best way I can think of the get the accidental in place is to split the
written note in half, i.e. make a quarter into two eighths. Then select the
second note and use the symbols dialog to add an accidental at that rhythmic
position. This should put the accidental at the middle of the trill. Then
you need to press Ctrl+UpArrow a few times to push the accidental above the
trill. The number of presses depends on the Y position of the trill. Trills
are normally created at Y=4 and the accidentals at Y=3. So, two or three
presses should do it. Finally, go back to the principle note and change it
back to its full length.

You also need to specify which note the trill begins on. The easiest way to
do this is to add a grace note before the principle note. You can even put
it in brackets if you want to make absolutely certain that the player won't
play it as a grace note before the beat.

Whew! That's disgusting. I think I need to look into writing a plug-in to do
all this in one operation.

Additional information from Dale on placing grace notes before the note:

Yes. Here's where it gets rather more complicated. Since the grace notes
are inserted BEFORE the note: 

1. Create a 2nd voice (alt_2). On the last beat of that 2nd voice,
divide it into small rests, such as 32nd perhaps.
2. Be sure all of the rests in this work-around voice 2 are hidden
(ctrl_shift_h on each)
3. Now insert your grace notes before the last of those rests, but do
not hide them.

You'll probably have to verify correct spacing via a sighted assistant.
But this is the work-around I got a few years ago from the Sibelius
forum.