There is a good walkthrough of setting up family sharing here. Please decide who is going to inherit the original Apple ID and who has to set up a new Apple ID by clicking here. Then log the other user out of that original account on all of their devices and have them log into the new Apple ID on all of their devices. Whoever has the Apple ID recovery tied to their cell phone number should inherit the original Apple ID that you have been sharing.
Once all of that has been done then you can follow the directions here for Family sharing, and don’t forget if you have many apple subscriptions then an Apple One account might be in your best interest.