1.1 Download image
To download a image go to http://www.debian.org/devel/debian-installer/ and copy the download link of the netinstall iso for your architecture from the netinst section (Right Click > Copy Link Location (Firefox) / Copy Link Address (Chrome)) and download it with the command
wget -c [copied link]
(you can simply paste the link with the shortcut STRG+SHIFT+V). Create now a file for the checksum with the command
in the same folder and open it with
Then click in the CD section on your architecture and click on the new page on SHA512SUM and copy from there the line of the netinstall iso (normally it is the second one from the bottom) into the sha512sum file (you can simply use the shortcut STRG+SHIFT+C to paste it). Save now the file with the shortcut STRG+SHIFT+O and close it with STRG+SHIFT+X. Finally check now the iso image with the command
sha512sum -c sha512sum
Normally the image should be fine, if not, try again to download it.