Plato's tale of descent is the Myth of Er which I shall condense from the last chapter of his Republic:
The souls are all hanging around in a mythical world, having arrived there from previous lives, and each has a lot to fulfill. This lot is also called a portion of fate (Moira) that is somehow representative of the character of that particular soul. For instance, the myth says the soul of Ajax, the intemperate and mighty warrior, chose the life of a lion, while Atalanta, the fleet young woman runner, chose the lot of an athlete, and another soul chose the lot of a skillful workman.
"When all the souls had chosen their lives according to their lots, they went before Lachesis [lachos = one's special lot or portion of fate]. And she sent with each, as the guardian of his life and the fulfiller of his choice, the genius [daimon] that had been chosen."
Lachesis leads the soul to the second of the three personifications of destiny, Klotho (klotho = to twist by spinning). "Under her hand and her turning of the spindle, the destiny of the chosen lot is ratified." (Given its particular twist?) "Then the genius [daimon] again led the soul to the spinning of Atropos [atropos = not to be turned, inflexible] to make the web of its destiny irreversible.
"And then without a backward glance the soul passes beneath the throne of Necessity," sometimes translated as the "lap" of Necessity.
The Platonic myth says the soul descends in four modes - via the body, the parents, place, and circumstances. These four ways can be instructions for completing the image you brought with you on arrival.
First, your body: Growing down means going with the sag of gravity that accompanies aging. ([Josephine] Baker told people she was sixty-four while she was still in her mid-fifties; she wore old clothes and gave up covering her baldness.)
Second, admitting yourself to be one among your people and a member of the family tree, including its twisted and rotten branches.
Third, living in a place that suits your soul and that ties you down with duties and customs.
Last, giving back what circumstances gave you by means of gestures that declare your full attachment to this world.
The Soul's Code: In Search of Character and Calling, p.44, 45, 62