How to create an new AppleID on an iPhone 6

My wife (iphone 6s, 15.8) and I (iphone 7, 15.8) have been sharing my AppleID account for years. I know this is supposed to be a no no, but we've not had any problems until now. Somehow imessaging is all screwed up. She's getting all my messages and I'm getting all her messages and I don't see any easy way to fix. So I'm ready to create a new AppleID for her.

Could someone please tell me the easiest way to do this. I have seen a suggestion to use the app store by creating a new ID rather than logging in, but loggin in is the only option. I will be replacing the 6s since it is no longer supported and the battery is shot, but I would like to fix this problem before replacing the phone.

Thanks for your help.

David