March from Velis Vel

"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."

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

Because the lands aren't entering the battlefield when they become copies of the creature, any "When [this creature] enters the battlefield" or "[This creature] enters the battlefield with" abilities of the copied creature won't apply.

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.

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

If the copied creature is copying something else, then the lands become copies of whatever that creature copied.

The lands copy exactly what was printed on the original creature and nothing else (unless that permanent is copying something else; see below). They don't copy whether that creature is tapped or untapped, whether it has any counters on it or Auras and Equipment attached to it, and so on.

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.

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

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.