Insert the USB drive. When Windows Explorer opens, right-click on the drive letter and choose Format. Unless you have a reason to doing it differently, choose exFat with 4,096 bytes allocation unit size.
Right-click on the drive letter and choose Properties.
Click the ReadyBoost tab. For simplicity, use the entire USB drive by clicking "Dedicate this device to ReadyBoost".
Click OK and the drive will be setup for ReadyBoost.
If you are watching Disk Management in Computer Management, drive space will drop to 1% free.