A letter journal can be approached in a few ways. You can begin your letters in the journal then rewrite on fashionable paper before you send them. The journal allows you to keep a collected copy of the letters you wrote as well as provide a reference for the last time you wrote someone.
Another method I propose is a letter journal to be read upon your death. In it you write to all the people in your life and say the final things you've always wanted to say but never have or to reinforce your feelings toward them. These are for all the things we wished we said more often. Or the final thoughts and blessings etc. we want to leave behind. This is meant to be filled with love, not hate. If you want to keep a letter journal to write all the bad things feel free. Anger journals are a good release but can hurt a lot of people unintentionally.
In the first couple pages, work on a list of the people you'd like to write to. As you finish each letter you can put a tick beside their name. Use the tear sheet journals so they can be taken out easily and given to the desired person when necessary.
Put a post-it on each letter with the current address so the person taking care of your affairs at least has an idea of how to contact the person. I recommend a post-it so you can change it if you need to and so it won't take up space in your letter. When the book is filled be sure to add it to your list of obligations to be taken care of in your will. You also want to store it in a safe place where it can be found. Where you keep your other important papers is good.
Happy letter writing!