Psalms and Gloria Patri in Introit Antiphons

I am doing an assignment and need some insight on what to look for:

1. How is a psalm assigned to an antiphon? i.e. if I am dealing with Signum Magnum Introit Antiphon from the Assumption of Our Lady, how was psalm 97 (98) assigned to it? Is the psalm related to the feast in some way? If yes, how? Where can I find such information?

2. I found the 8 tones of the Gloria Patri of the Antiphons. Where can I find more information on their make up?

Thank you loads for your help.