It's really hard to try and land a sing, unless you ledge-cancel it, which is when you sing right above the ledge and cancel the amount of time she sings by catching the ledge. If you use it any other time, you can expect to be punished. Rest is just an attack that has really high knockback and comes out quickly, but obviously the wind-down lag is horrible. There are ways to combo into it, using jiggly puff's bair, nair, and fair.