Get inside the groove.

Click empty cells to add notes. Click filled cells to inspect. Double-click or F2 edits velocity. Drag across to paint, vertically to adjust velocity. Press B for the draw tool (drag in any direction to paint) and E for the erase tool (click or drag to clear); press the key again or Escape to return. Right-drag across filled notes to erase; right-click a filled note to subdivide it, ramp its velocities, edit or clear it; right-click a track or empty cell to fill this bar, rename the track, change its color or choose a sample. Escape cancels a gesture.