You both need to be part of a Family Sharing group. Read Set up Family Sharing
Once one of you has set it up the other needs to be invited to join the Family Sharing group. Read the section "Join a family group" in the article "Set up Family Sharing". This link will take you straight there --> https://support.apple.com/HT201088#join
The person being invited needs to:
1. Open the Settings app, then tap your name.
2. Tap Invitations.
3. Follow the onscreen instructions.
After that the process should be fairly automatic.
From https://support.apple.com/108783
"If a family member is already paying for their own iCloud+ plan, they can choose to switch to your plan or keep their own and still remain part of the family. When they switch to the shared family plan, they get a refund for the remainder of their personal plan. They can't keep their current plan and use the shared family plan at the same time."