I want to add memory to this phone. What steps are necessary?

Adding storage to a phone is not possible. If you need more space, you will need to delete something.

it's perfectly explained here: https://support.apple.com/en-us/HT201265

you cannot add physical memory to iphone, but you can:

1. delete unused apps
2. delete unnecessary photos
3. delete safari cache
4. delete history in messengers/chats etc
5. use icloud for photos, start here: https://support.apple.com/en-us/HT206504