Three pillars; body, mind, environment.
Body - A varied diet including lots of plants and a mix of proteins— with 5 or 6 figures to spend a month you never need to eat another low-quality convenience meal. An appropriate exercise routine for the subject's level, incorporating at least some light strength training and adding modules as the habits become ingrained and sustainable (candidate exercises include yoga, jogging, crossfit, and kickboxing in no particular order). Sleep hygiene— 6-8 hours on a consistent schedule according to the subject's needs. Quit smoking entirely if applicable and reduce other recreational drug consumption with option to eliminate entirely if also needed.
Mind - Shop around for therapists; school matters less than rapport. Key skills to develop involve developing awareness of one's own emotions, plus the ability to nonjudgmentally moderate & process those emotions. Identify and excise any behavioral addictions; smartphones, social media, and some video games are high-level targets for excision. Meditation and mindfulness may be productive.
Environment - Ensure the subject has 1 to 3 close friends/loved ones to mutually confide in, plus several more casual friends. Connect with at least one local community (church is the archetypal answer but rec sports, bar trivia, meetup groups, clubs, volunteer orgs, or local gaming scenes can all be good choices) and make as many in-person appearances as the subject can comfortably handle. Ensure physical spaces are comfortable; helpful targets to look into will likely include tidiness, illumination, and ventilation. Minimize exposure to unwanted stimuli such as traffic noise.
Tie all of the above together with a healthy dose of slack and I think you'd be well on your way with most subjects barring clinical mental health issues.
Links I've been collecting for when I get around to trying to ansewr this question for myself:
* "MDMA Solo" https://news.ycombinator.com/item?id=25974701
* https://news.ycombinator.com/item?id=19997984
* https://sashachapin.substack.com/p/how-i-attained-persistent-self-love
* https://twitter.com/m_ashcroft/status/1486398240643760133
* https://www.lesswrong.com/posts/ZbgCx2ntD5eu8Cno9/how-to-be-happy
* https://www.lesswrong.com/tag/well-being
* https://www.lesswrong.com/posts/tEDXpFgsHsm5T8sWz/app-and-book-recommendations-for-people-who-want-to-be
* that alicorn luminosity sequence
* https://www.lesswrong.com/posts/BfTW9jmDzujYkhjAb/you-are-probably-underestimating-how-good-self-love-can-be
* https://kajsotala.fi/2017/07/how-i-found-fixed-the-root-problem-behind-my-depression-and-anxiety-after-20-years/
* https://www.lesswrong.com/posts/j3QDAqSQFi4BpHxQK/a-guide-for-productivity-2
* https://www.lesswrong.com/posts/iYR9hKzTKGhZwTPWK/meaningful-rest
* https://alexvermeer.com/life-hacking/
* https://www.lesswrong.com/posts/WLSJHJzRjLjRuQ3us/how-feeling-more-secure-feels-different-than-i-expected
https://qualiacomputing.com/2021/04/04/* buddhist-annealing-wireheading-done-right-with-the-seven-factors-of-awakening/
* https://forum.effectivealtruism.org/posts/i2Q3DTsQq9THhFEgR/introducing-effective-self-help
In May of 2022? I would consider it a high priority to unplug them from all sources of bad news about current events. This could probably be done most effectively by doubling down on a hobby which they already value, and taking them "off-grid" in a moderately sized group of supportive and relatively values-aligned individuals. The cash could be used to fake some sort of scholarship or fellowship award to them to basically pay them to do something they want to do more of, and remove them from whatever employment is probably making them unhappy already.
From my personal experience, I would have them take up one or two competitive arts.
Timing yourself to improve your personal best at, say, running, does not count. Running on a track against, or in a longer race against, a small handful of people that you can potentially beat does count, although I would lean toward recommending something where you have to deal with the counter-moves of your opponent. Boxing or kickboxing training that includes sparring against others counts; doing boxing training at a gym that does not do sparring doesn’t count. Playing chess or Go counts. Basketball, hockey, soccer, etc, all count. Playing online competitive video games technically counts under this definition, but I’m excluding it; those mostly make me feel bad.
Having an opponent that will challenge you with counter-moves, and do their best to get one over on you, but who you can beat if you train and try hard enough, has no substitute. Winning against someone who has put everything into the fight gives confidence that you can apply all over the place. Plus it feels great.
My experience: I’ve spent the past two years running and lifting. These mean I look great physically, and am healthy, and get the exercise endorphins and stuff. But they didn’t meet the competitive need! I’ve recently gotten back into boxing at a sparring gym. The competitive aspect of being in the ring, trying to best the other guy, is something I’ve really been missing. It also directs my training at a real concrete purpose, instead of the colder “increase the weight / increase running speed” metric-tracking approach to those forms of exercise.
I also play Go, and it used to serve this purpose well in my life.
(This competitiveness stuff might be more important for men than it is for women - I’m not sure. I’d definitely give this advice to a man, and I’d give it as a ‘maybe’ to a woman. Of course, women can get a lot of value from competition; I’m just not sure if the lack of it would gnaw at them the way it was gnawing at me.)
For how long? Peaceful can be achieved easily by death, joyful is more tricky. Shortcuts to joy, especially with a bar of "as joyful as the most joyful I've ever met", are pretty short-lived - I guess I'd try drugs or other wirehead-like intense experiences, but I don't think this is the intent of the question.
If I've got 6 figures per month to spend, guaranteed for the life of the target, I think that knowledge goes a long way toward removing a lot of barriers to joy. I'd then focus on triggering joyous feelings, in hopes that the target can learn to repeat it fairly often.
I tend to think of joy/satisfaction as very slow and difficult to change in adults - not quite a set-point, but habits and beliefs created and reinforced in childhood tend to stay. Which means I expect to fail in this goal for the vast majority of random people.
What general methods, tools, approaches might you take?
Certain kinds of therapy, certain kinds of meditation, certain types of medication, certain types of coaching, certain activities, certain processes, and so on. Specific things, and/or meta things.
For constraints, imagine that you can spend 5 or 6 figures of US dollars per month, you're there in person full time with them. You don't know who it's going to be, but you know they don't have any notable medical conditions or past history. They're a semi willing participant. You can try anything that exists as of May 2022.