Using debian-8.7.1-amd64-xfce-CD-1.iso burned to a USB flash drive:
1. Power down computer. Insert USB flash drive. Power up computer. Boot the USB flash drive with the Debian installer (you stated you already know how to do this).
2. "Debian GNU/Linux installer boot menu" -- choose "Advanced options".
3. "Advanced options" -- choose "Rescue mode".
4. "Language" -- choose your preferred language.
5. "Country, territory or area" -- choose your preferred country, territory, or area.
6. "Keymap to use" -- pick your preferred keymap.
7. On my laptop that I am using for testing, I am prompted to load missing firmware. I chose "No".
8. I am also prompted to choose the primary network interface. I chose "eth0".
9. "Hostname" -- accept default (debian).
10. "Domain name" -- leave blank
11. "Select your time zone" -- choose your time zone.
12. "Device to use as root file system" -- choose "Do not use a root file system".
13. "Rescue operations" -- choose "Execute a shell in the installer environment".
14. "Executing a shell" -- choose "Continue".
You should now be presented with a root prompt. Do your work. Enter "exit" when you're done.