Can you cast a creature with flash during the declare attackers step of your own turn?

Flash (CR 702.8a) grants permission to cast a spell "any time you could cast an instant." During your own declare attackers step, the game is in the combat phase, and players receive priority during this step — meaning you can cast instants (and therefore Flash spells) at any point when you hold priority.

After you declare attackers (CR 508.1), both players receive priority in turn order (CR 508.3). At that point, you may cast a creature with Flash. Similarly, before you even declare attackers, you receive priority at the start of the declare attackers step (CR 508.1), giving you another window.

Keep in mind the creature will enter the battlefield after attackers are already declared. It will NOT be able to attack that turn, since it wasn't declared as an attacker during the declare attackers step (CR 508.1), and it will have summoning sickness unless it has Haste (CR 302.6).

Example: You attack with a 2/2. Your opponent passes priority back to you in the declare attackers step. You cast Briarbridge Patrol (which has Flash). It enters the battlefield, but it is not attacking — it simply enters as a non-attacking creature and will have summoning sickness next turn unless it has Haste.