OMG, shes wriglling so much when i bathe her...!! how do i stop her?

My 3 month old babyb is wriggling each time i bathe her...and its scaring the F$^&*^ out of me... sometimes I let my daughter bathe her under my "supervision", shes 9 and she seems more confidant than I am... i've read the whole article where u gotta hold the neck, head and spine.... but still... how do i get her to hold still?