How do you get the flash in Lego batman 2?

You can't actually get The Flash in Lego Batman 2: DC Super Heroes. He's not a playable character in the game.

However, you can play as him in the game's "Free Play" mode after you unlock him by:

1. Completing the main story mode of the game.

2. Finding all the "Minikits" in each level, which will unlock the "Free Play" mode for that level.

3. Selecting The Flash as a playable character from the character selection screen in "Free Play".

He's a really fun character to play as in "Free Play" because of his super speed!