I think it is normally quite automatic. After samādhi, if your mind inclines to the recollecting of past lives, it will tend to go there. It all unfolds quite naturally and there is not much you have to do. Whether your mind inclines there or not will in turn depend on your conditioning, such as how important you think it is. The standard passage in the suttas on the recollection of past lives, found e.g. at MN 4, is as follows:
When my concentrated mind was thus purified, bright, unblemished, rid of imperfection, malleable, wieldy, steady, and attained to imperturbability, I directed it to knowledge of the recollection of past lives. I recollected my manifold past lives, that is, one birth, two births, three births, four births, five births, ten births, twenty births, thirty births, forty births, fifty births, a hundred births, a thousand births, a hundred thousand births, many aeons of world-contraction, many aeons of world-expansion, many aeons of world-contraction and expansion: ‘There I was so named, of such a clan, with such an appearance, such was my nutriment, such my experience of pleasure and pain, such my life-term; and passing away from there, I reappeared elsewhere; and there too I was so named, of such a clan, with such an appearance, such was my nutriment, such my experience of pleasure and pain, such my life-term; and passing away from there, I reappeared here.’ Thus with their aspects and particulars I recollected my manifold past lives.
As you can see from this, the recollection is both detailed and structured. What you see is how one life leads to the next, eventually taking you to your present existence.