How to use Arduino to create a programmable keyboard?

I want to connect a USB keyboard to my Arduino and then connect my Arduino to my computer via USB so that my Arduino is recognized as a keyboard. This will allow me to remap the keys.

Required hardware for this setup:

[Keyboard] -> [Arduino (As Virtual Keyboard)] -> [Computer]

The goal is to achieve this configuration as cheaply as possible. 😀

• Arduino Pro Micro, that uses the ATmega32u4 chip, and that can do communication by USB as a virtual keyboard.
• USB host board like this to get ANOTHER USB connection, to which I can connect the real keyboard.