Hi there, I would advise to only use the sippy cup for water or juice for practice. It won’t be till about 12 months before baby is able to control and hold a sippy cup on his own. If milk bottle flow is too slow, switch to a more high flow teat or make the opening in the teat larger. This article may help https://www.parents.com/toddlers-preschoolers/feeding/the-7-best-transitional-sippy-cups-and-when-to-introduce-them/
Below article may help you.https://sg.theasianparent.com/choosing-best-sippy-cup-toddler