After powering on the HDD, you would insert the USB B into the HDD, and USB A into your PC.
If you are just using it for storage, you would simply save the PS2 ISO to that drive location.
If you are using it to install and play games to it, then you would install FHDB first, then use a PS2 ISO installer. After, you would remove the HDD from the chassis and insert it into the Fat PS2, followed by a Network Adapter.
If you have a PS2 Slim, you would need a memory card with FMCB. You would install PS2 ISOs with USButil, and connect the HDD and chassis to the PS2 USB port.