Press and release a function button (top row) and then press any other button to execute a function (switch tracks, change volume, set effect, etc).
For example, press Fnc1 (led should light up), and then press any of buttons in the lower 3 rows to change instruments.
Press Fnc3 and then any of the top row buttons to switch tracks.
Tracks are quantized to 8 notes per bar, patterns can be set to 4 or 8 bars each, 4 total patters are available in "Song" mode, Fnc4 + D# to switch between song and pattern modes.