To get it on to an external drive, your photos need to get to a computer first (because an external drive can not be plugged into an iPhone).
To get the photos to a Mac, you can use a standard iPhone to USB lightning cable to plug your phone into the computer. Your Mac will normally pop up asking to import the photos, accept this. (Windows computers might be capable of this, I am not sure.)
Or you can use iCloud Drive for any computer. On your phone, turn on iCloud Photo Library in settings. When your photos are uploaded to the cloud, go to iCloud.com on your computer. Open the photos web-app, select your desired photos, and at the top, you should see some buttons to import the photos to your computer.
Once the photos are on your laptop, you can move them to your desired external drive.