March from Velis Vel

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.

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

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

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

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.