My opponent declares two creatures as attackers. I want to play Act of Aggression to gain control of one of their creatures and so I can have it block their other creature. Is this possible? Is so, during what step do I need to play my Act of Aggression? If I played it during my declare blockers step, would the creature that I now controlled still be attacking me?
Yes, that's fine. You would play the spell during the declare attackers step, after attackers are declared, but before proceeding to the declare blockers step. Then you are free to declare it as a blocker. If you wait for your declare blockers step, it's too late to block with the creature.
The ordering is:
Any time a creature changes controllers, it is automatically removed from combat, so it will no longer be considered attacking you (or attacking/blocking at all if it previously was).