Entering chords and melodies

Despite having watched the tutorials a few times, I'm still very confused about entering chords/melodies in the same measure. I inevitably end up with issues such as...
Bottom line - I guess I just need a clearer step-by-step tutorial, since I'm clearly misunderstanding the input mechanics. Thanks!
- stems going down when I want them to go up (I'm writing a piece for a bell choir)
- not being able to create a melody line which, like the chords, has multiple harmonies (again, since it's a piece for bells, there are far more notes in each staff than is typical of a piano piece)
- not being able to modify the previously-created chords or melodies effectively once I've moved on to other measures
Bottom line - I guess I just need a clearer step-by-step tutorial, since I'm clearly misunderstanding the input mechanics. Thanks!