How do I craft command blocks?

Like Caleb said, command blocks are only on the PC/Mac versions of Minecraft.

You can't craft a command block and can't even obtain them through creative mode. You have to enter this command in order to spawn one in:

/give [player] command_block [amount]

They basically execute a command when given a redstone signal.

Unfortunately Command blocks are not available on the iPad.

Command blocks have only been implemented on the PC/Mac version of Minecraft, they are not in the Pocket Edition or Console Edition of the game.

As of update 1.0.5, they have been added to the native game. They can only be obtained via commands in PE.

Command Blocks can only be obtained by using Commands. They are not available in the Creative Inventory, and cannot be obtained naturally in Survival Mode.

To obtain them, use this command:

/give @p command_block

Additional Wiki source.