i would like to add my mothers letters to my father to memories
If you are just wanting to preserve your mother's letter to your father, in the Memory Gallery in Family Search, upload the image to your Memory Gallery. How do I upload memories to FamilySearch? Once it is in you Gallery , from there you may attach it to both the record of your father and your mother. How do I tag memories of my ancestors or relatives in Family Tree?
You may want to upload them in order, or distinguish them by date written. They will be a good snapshot of what was happening for them at that time.
We wish you success in your efforts,