select your rectangle tool,
cancel-stroke/outline (from top of colorchart),
and select white as fill.
¤ Draw the whitekey
on the stage, just to an approximate size.
Size your drawing
to an exact 15x80 with Info screen [ Ctrl + I ].
While whitekey is selected, copy to clipboard
[ Ctrl + C ] for other buttons later.
Move this first whitekey to the left edge of the stage. Buttons will be automatically aligned later so you need not worry about exact position.
With this first whitekey still selected,
press [ F-8 ] to open Convert to Symbol
, check: Button
. This really doesn't need a name.
The result is your first Button Symbol on frame 1 of layer c3.
we will add sound to "over" and "down" button states,
plus a downwards-nudge to indicate a "hotspot".