How does the Stoic imperative of 'living according to nature' lead to tranquility?

What is meant by 'living according to nature'?

It involves:

Reflecting on the nature of humans

To be honest in this day and age, you must take a Evolutionary perspective and see humans as highly evolved animals. You must remove all Political Correctness and other cultural indoctrination from your understanding. You don't need to be an expert on human behaviour, but you must respect scientific fields such as biology and neuroscience. Although more speculative in nature, evolutionary psychology is also insightful. Avoid Politically-Correct fake 'science' like Sociology.

Understand why we were created and our place in society 

Having reflected on human nature, you will have an understanding of human instincts and motivations. You should realise that though we may try, human beings cannot overcome their nature, but they can pervert it, unlike other animals. This is owing to humans susceptibility to indoctrination though their advanced cognitive abilities.

With a Darwinist perspective, you will understand that human being are made - through evolutionary processes - to (1) reproduce and (2) seek social status by benefiting their community.

Reproduction

You will find and attract a suitable mate and raise children. Unfortunately, nowadays many indoctrinated people see children as an inconvenience to their desperately hedonistic, consumerist lifestyle. Such people live in denial and dishonesty, and end up as jaded cat ladies or grumpy old men.

Seek social status by benefiting their community

Human beings are social animals, as created through evolutionary processes, and seek acceptance and praise from others in their community. It is possible for a few to gain fake praise as a social media star, virtue-signaller or celebrity, but these are hollow, fragile and unfulfilling.

Real, solid, unshakable social status is gained by serving your community and being excellent about it. Being excellent in your role is what the ancients meant by virtue.

The ways in which one can benefit their community are numerous. It could be as leader (politician), care-giver (doctor/nurse/ambulance), protector (police/fire), child-rearing, etc. It is usually easier to gain social status by being in the public eye and serving people directly. It takes more cognitive work to justify back-office and managerial jobs - if indeed these jobs actually serve the community and are not just 'bullshit jobs'. Of course, employment is not the only way to serve your community. You can be retired and contribute to charities, etc.

Unfortunately, some people won't always recognise the value of your profession. But when you are a Stoic and have cultivated your appreciation of objective virtue, lack of recognition and criticism will not bother you. You will recognise that others judgements are outside your control and you will be content in knowing that your behaviour is in accord with nature and virtuous. Fortunately, most people will recognise your contribution to the community and appreciate it!

So how does the 'living according to nature' lead to tranquility?

Now that we know what 'living according to nature' means we should have some clues as to how it can lead to tranquility.

Chiefly, the Stoics emphasise recognising what is in your control and what is not.

The Stoic recognises that he if he acts according to nature, beneficial to the society, he is likely to receive the social status he craves. Though, if someone criticises him unjustly, he can ignore the criticism, as the critic is deluded and not living according to nature, and not beneficial to the community.

The Stoic recognises that all he can do is live authentically according to his nature. His only job is this and where he deviates from this, he can use reflection and meditation to remind himself of his true nature and follow the straight path again.

He does not seek to control things that he can't, because he recognises the reality of the situation. Having accepted his lot and having done his work excellently, he can relax, be proud and grateful for his lot in the world. This creates a feeling of resounding tranquility.