You don't.
But the songs are already assigned to keys.
The alpha-pad number keys 1 to 8 are assigned to the 8 different forms, 9 and 0 are for bind song and shield song.
X reverts back to song form, (the same as the number-1 key) WASD swims in 4 directions (but the mouse is more accurate) R automatically makes you roll through the water, for drawing stuff towards you, or shifting between dual form forms, the space bar propels you forward with more force, or jump from a wall.
Tab brings the map, exc the little menu where all the cooking takes place, and you find you pets and treasures.