Instead of making a post full of screenshots showing good flash positions I am going to try and simply lay down a few rules to follow for making the most of your flashbang (now more important than ever since by default GO does not let you carry 2 – as Source / 1.6 did).
So I guess the first rule is to not waste it. If you absolutely must pre-nade then it may as well be a HE grenade that you throw. If the enemy hear a couple of flashbangs come flying from a chokepoint, they are going to know that they won’t get flashed if they now rush that site. A flash is most useful either the second before an enemy team pushes onto your position, or as a counter flash to an enemy grenade.
That roughly covers when to use one, but what about how? A perfect flashbang is one that lands right infront of the enemy, yet gives them no chance to react to it. In other words; as soon as the grenade enters the enemies field-of-view it should explode. This gives them no chance to look away, and pretty much guarantees that they will get blinded.
To achieve this it is best to make use of walls and obstacles to bounce the nade off. Go in a server and sit at your preferred position for defending a bombsite and work out the timings and positions for a good flash. If you have someone to help you, and act as the enemy rushing you in this situation, then great.
Using flashes in pairs is another great defensive tactic. In this case, one player calls out that he is flashing, then, just after the flash goes off, the other player can quickly peek out and try and get some kills. If there is still time left this player can then throw a flash, and the other teammate gets to peek out. If you get the timing really perfect then you can peek out just as your teammate throws the grenade, possibly catching a few enemy players as they try to duck away. If you do this quickly you can basically get an extra peek, since you peek before the grenade explodes, and then after.
When you are attacking a bombsite, flashes are best used by throwing them into areas where you suspect the enemy to be hiding, and then rushing in to kill them whilst they cannot see.