How do I install "non-free firmware" (wifi driver) from USB? (Debian)
See the relevant section in the installation manual.
To answer your questions in more detail:
- You'll find the files on http://cdimage.debian.org/cdimage/unofficial/non-free/firmware/; if you're installing Debian 8, look in http://cdimage.debian.org/cdimage/unofficial/non-free/firmware/jessie/current/ and download whichever archive is the easiest for you to handle (presumably firmware.tar.gz or firmware.zip).
- Extract the firmware archive in the root of a flash drive; plug the drive into the computer you're installing, and the installer should find the firmware automatically.
- The typical FAT32 filesystem is just fine. If you've got a larger drive using ExFAT, I'm not sure the installer will be able to use it...