Tranquillity, for the Stoics, is not a mood you cultivate directly. It is what is left when you stop assenting to the judgements that disturb you - the byproduct of getting other things right, rather than a target you can aim at. Seneca devotes a dialogue to it, diagnosing the restlessness of people who change houses, cities and occupations while carrying the actual problem along with them. Marcus keeps returning to the retreat available at any moment: not a country estate, just his own mind.
— Marcus Aurelius, The Meditations 7.33"The understanding, too, preserves its own tranquillity by abstraction, and the governing self does not grow worse; but it is for the parts which are injured by the pain, if they can, to declare it."
— Marcus Aurelius, The Meditations 5.02"How simple to reject and to wipe away every disturbing or alien imagination, and straightway to be in perfect calm."
— Marcus Aurelius, The Meditations 8.36"Do not allow the imagination of the whole of your life to confuse you, do not dwell upon all the manifold troubles which have come to pass and will come to pass, but ask yourself in regard to every present piece of work: what is there here that can't be borne and can't be endured?"
— Marcus Aurelius, The Meditations 2.05"You see how few things a man need master in order to live a smooth and godfearing life; for the gods themselves will require nothing more of him who keeps these precepts."
— Seneca, On Clemency 2.05"Sorrow overwhelms men's minds, casts them down, contracts them: now this cannot happen to the wise man even in his greatest misfortunes, but he will beat back the rage of Fortune and triumph over it: he will always retain the same calm, undisturbed expression of countenance, which he never could do were he accessible to sorrow."
— Marcus Aurelius, The Meditations 4.24"For this brings not only the tranquillity from doing right but also from doing few things."
— Seneca, On the Happy Life XXVIII"Why, though you are hardly aware of it, is there not a whirling hurricane at this moment spinning round and confusing your minds, making them seek and avoid the very same things, now raising them aloft and now dashing them below?. . . . . .""
— Marcus Aurelius, The Meditations 7.68"For, in the midst of all this, what hinders the mind from preserving its own self in tranquillity, in true judgement about what surrounds it and ready use of what is submitted to it, so that judgement says to what befalls it: 'this is what you are in reality, even if you seem other in appearance', and use says to what is given to it: 'I was looking for you, for the present is to me always material of reasonable and political virtue, that is (generally speaking) of the art of man or God'; since whatever comes to pass is suited to God or man, and is neither novel nor hard to deal with, but familiar and easy to handle."
— Seneca, On the Firmness of a Wise Man 1.08"As he presses forward and makes his way towards the life that is sublime, well-ordered, without fear, proceeding in a regular and harmonious course, tranquil, beneficent, made for the good of mankind useful both to itself and to others, he will neither long nor weep for anything that is grovelling."
— Marcus Aurelius, The Meditations 7.28"Withdraw into yourself: the reasonable governing self is by its nature content with its own just actions and the tranquillity it thus secures."
The recurring practical claim is that most disturbance is self-supplied and can simply be dropped, and the recurring practical advice is to do fewer things. Marcus makes both points in the same breath - the calm of doing right, and the calm of not doing much. Read alongside the anger and judgement topics, this one is best understood as the destination rather than the method. Nothing here tells you to relax. It tells you what stops needing to be defended once you have sorted out what is actually yours.