When she signs into her ID she will be presented with options to keep and merge data that already exists on the phone into her account and her iCloud, retaining what is on the phone already. That is what she wants to do so say yes when asked. Note that this will include whatever of yours is on the device, so you and she will want to make sure any information that you do not want her keeping on her phone is deleted. Since the phone will then be signed into her account, deleting items won't have any effect on your account.
Most things should remain on the phone but any apps purchased with your account can't be updated with her account. If she doesn't want to buy them again you will need to share what can be shared with Family Sharing. To learn how to set up Family Sharing, read this support article --> Set up Family Sharing - Apple Support
It is possible some things that aren't part of iCloud won't continue to work. I think this might be things such as game scores which are kept as part of your account she is no longer using.