Pain wins our best writing. The hard chapters are documented in detail, with their dates and their dialogue, written in the mind and rewritten and recalled so they can hurt us again - while an afternoon of pure happiness is kept in a single line: it was a lovely day. This theme reopens the file. The finding behind it is simple: write the intensely positive experience at its full length, and give the senses priority. You should know that joy can be lived again, and many times over.
Choose one moment. Not a good era but a moment, the size of a scene: the phone call with the good news, the kitchen on that holiday, a few seconds of riding a bicycle into the sunlight, a time when everything seemed perfect. This method will take any memory, even one from thirty years ago - the archive does not expire - but here we encourage what is more recent, and best of all something from your last day.
Write as if you were inside the moment again, in the present tense - I am standing, not I stood. Give the senses priority: the warmth of the air, the exact light, what you are holding, what you are wearing, the sounds in the place, who was there and what they were doing. And go much more slowly than real time. Perhaps the moment lasted forty seconds; nothing stops you writing it as if it lasted twenty minutes. Speed is how it was lost the first time.