Rename the third track to "Melody," or something like that.
Change the track's program.
Look at the "Instrument Parameters" box in the "Special Parameters" toolbox on the left side of the main editing window.
Make sure that you have the "Melody" track selected.
Select the "General MIDI" bank, which is probably already selected.
For the program, select whatever you prefer. I decided to use program 51, called "Synth Strings 1." It reminds me of music from the 1980s.
After setting the program, press 'play' on the transport toolbar, and let it go for just a second. This will allow Rosegarden to send the program-change message to FluidSynth. It isn't strictly necessary, and it would have been done later anyway, but doing it now helps to avoid confusion later.
Use the "Select and Edit" tool to create a segment in the "Melody" track, of four measures long.
Edit the segment in the default editor by double-clicking on it.
Create four measures of a melody. It doesn't have to be complicated, or even interesting.
Close the notation editor, and listen to the the three tracks. Don't forget to reset the playhead to the beginning of the session!
It sounds a bit silly to have the melody enter at the very beginning, so add an introduction. With the "Select and Edit" tool (press 'F2' on the keyboard to engage it), click-and-drag the melody segment to a few bars or beats later. Note that, even if you move it to start on beat 2, 3, or 4, the view in the notation editor will always start the segment on beat 1.
