time signature
Turn off measure numbers: Format → Style → uncheck Measure Numbers
Turn off page numbers: Format → Style → Page
key signature
add notes
It likes to jump octaves when you least expect it. ctrl+up and ctrl+down move the currently-selected note up or down an octave.
Tuplets: In Note Entry mode, select a note the total length of the tuplet. (So, for 3 quarter note triplets, select a half note.) Push ctrl+3, then enter the three notes.
Fermata: Select the note that gets the fermata. From the “Articulations” palette on the left, double-click the fermata.
pickup notes
add slurs
delete extra measures
set page to A7, landscape
Futz with the spacing until you're happy
To split a measure in the middle:
Select the note immediately after where you want the split
Tools → Measure → “Split Measure Before Selected Note/Rest”
Select a note in the measure before the split
press enter; the lines should be how you want them now.
400% zoom in Musescore 3. 66.6% zoom in GIMP.
Fix white background; it's not #FFFFFF.