For the letter, would you want to seal and keep it or save it for your moon phase of choice (hehe, sorry; I'm a nerd)? If you'd want to keep it, I'd say just write the note, squish the herbs in between the paper while folding it, and then seal it in an envelope and keep it somewhere private. If you'd want to burn it (aka releasing your letter's energies into the universe), then do the same as I said and burn the envelope until the note is gone.
Does this help? Message me if you want! :) Merry Meet!