How can the Soul stone be returned?

How one can return the Soul stone ( obviously, not putting the stone on Vormir)

Well, we don't actually know how but it seems likely and we have no reason to assume otherwise that it can just be returned to Vormir.

Presumably, giving up the Soul Stone (perhaps by throwing it off the cliff) is enough.

Since Red skull is free after the Soul stone is given, so he can't return the stone to him.

There's no indication in Infinity War that the Red Skull is freed once the stone is taken.

A lifetime ago, I, too, sought the stones. I even held one in my hand. But it cast me out, banished me here. Guiding others to a treasure I cannot possess

His role is to guide not safeguard and he can guide from anywhere and, essentially, no time has passed for him since the stone was taken.

And if anyway he is managed to return the Soul stone, then he can also bring back Natasha ( as it's mentioned by Red skull: A soul for a soul).

No, that's not the way it works....there's no indication that returning the stone would bring Nat back to life...in fact loss of life isn't technically required, only that you lose what you love.

In order to take the stone you must lose that which you love. A soul...For a soul.

Granted loss of life seems implicit but it's not explicitly put that way. Hawkeye doesn't kill Natasha, she sacrifices herself and Clint loses someone he loves by letting her go....

Unless we actually see this event portrayed in a future movie, or the directors decide to offer an official explanation, we can't know for sure.

But recall the idea is that Captain America returns at the exact time the stones are removed. Professor Hulk describes it as meaning that, chronologically, the stone never left. So when he arrives, the stone has never left Vormir. So the condition for Red Skull's release may have never actually been achieved: the stone just moved around a bit, nothing more.

Some interpret Professor Hulk's explanation (who, by the way, you shouldn't entirely trust to know what he's talking about; it's Tony that figured out how to time travel correctly, not Hulk/Banner) as meaning that if correctly placed then there is no alternative timeline created, despite the evident and obvious disruptions: reality collapses them back in to each other somehow, or the apparent "alternate timeline" only exists to serve the purposes of the time heist and then ceases to continue on independently after the stones are replaced.

Alternatively, in the comics there's a certain sense in which the Infinity Stones have their own will and the capacity to enact it, if only subtly. If much the same holds in the MCU—and the tesseract specifically taking Red Skull to serve as gatekeeper to the Soul Stone, as well as Red Skull attributing a "certain wisdom" to the soul stone in particular, strongly suggests it does, and that the stones work together—, then the stone(s) may have specifically conditioned Red Skull's release not on a generic "someone gets the stone", but rather on "a particular someone with a particular goal they are likely to achieve gets the stone" condition. Or, less opaquely, it was specifically conditioned on Thanos acquiring the stone for the express purpose of killing off half the universe. Or, perhaps more easily, the stone on its return simply doesn't release the Red Skull because it knows his purpose has not truly been fulfilled.

Moreover, the stone could simply return itself to wherever it is it comes from at that time. How did you suppose it got there in the first place?

He used the tesseract. If Rogers snuck on the plane in Germany before it crashed in the Arctic, he could simply follow Red Skull through the portal to Vormir, he could use the Tesseract to get back to earth from Vormir, allowing him enough time to meet Peggy, and not be late for their dance.