You can cast a spell using flashback even if it was somehow put into your graveyard without having been cast.

To determine the total cost of a spell, start with the mana cost or alternative cost (such as a flashback cost) you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.

If a card with flashback is put into your graveyard during your turn, you can cast it if it's legal to do so before any other player can take any actions.

You must still follow any timing restrictions and permissions, including those based on the card's type. For instance, you can cast a sorcery using flashback only when you could normally cast a sorcery.

If the copied creature has  in its mana cost, X is 0.

Any "enters" abilities of the copied creature will trigger when the token enters. Any "as [this creature] enters" or "[this creature] enters with" abilities of the target creature will also work.

If the copied creature is a token, the new token that's created copies the original characteristics of that token as stated by the effect that created the token.

A spell cast using flashback will always be exiled afterward, whether it resolves, is countered, or leaves the stack in some other way.

The token copies exactly what was printed on the original creature (unless that creature is copying something else or is a token; see below). It doesn't copy whether that creature is tapped or untapped, whether it has any counters on it or any Auras or Equipment attached to it, or any non-copy effects that have changed its types, color, power and toughness, and so on.

"Flashback [cost]" means "You may cast this card from your graveyard by paying [cost] rather than paying its mana cost" and "If the flashback cost was paid, exile this card instead of putting it anywhere else any time it would leave the stack."

If the copied creature is copying something else, then the token enters as whatever that creature copied.