How to store photos in icloud to an external hard drive.

I have a iphone that has 12,000 photos in it and want to delete most of them but still keep them somewhere. So I bought an external hard drive, MY PASSPORT WD with 2T.

What I need is some very basic information as to how to go about downloading the photos from my camera or from icloud into this external device. I need a step by step approach. Please don't tell me to connect the external hard drive to the XYZ without telling me exactly how to connect it. I can follow instructions as long as they are complete instructions. Assume I know nothing. Thank you