How Do I Linked Second User With There Own Apple ID?

I am looking at purchasing the new iMac 24 inch, my first Mac computer. I will be setting it up for my wife and myself. I will set up my account when setting up iMac for the first time. After watching your tutorials I know how to set my wife’s user account up, but, I do not understand where or how to to link that user account up with her Apple ID. Of course we have separate Apple id’s as we both have phones.
I understand it is best practice that we both have are own accounts and that both accounts are linked to there own Apple ID. Correct me if I am wrong.
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Bill