Turning this feature on couldn’t be simpler. Just stick the flash drive, SD card, or whatever flash device you want into your computer. You’ll see this dialog pop up:

Click the “Speed up my system” link, and you’ll be taken to the ReadyBoost panel for the flash drive:

Just select Use this device, and set the amount of space you want to use. Windows will create what is essentially a paging file on the disk, and will use it for faster access.

Note that this will only work with USB 2.0 devices or SD cards.