You will need: 1) an ISO file with your copy of Windows 7 (all legal, needless to say, huh?), 2) a USB stick at least as large as the ISO file and 3) the Gparted program ("gparted" package in Ubuntu/Debian) and 4) support for NTFS ("ntfs-3g" package in Ubuntu/Debian).
Plug your USB stick, open Gparted, and select the USB stick in the devices menu. Take note of its identifier, for example /dev/sdb1. Unmount it, erase all the partitions, and create a single, primary, NTFS partition. Then use the Manage flags menu to enable the "boot" flag.
Then open a terminal and start an administrator session:
Set up the NTFS filesystem:
(replace sdXY with the identifiers you've determined above).
Then create a mount point and mount the USB stick:
Now mount your ISO image:
Copy the contents over:
Once finished (it might take a while), unmount the USB stick:
Unplug the USB stick, then plug it back in and mount it again:
Now let's make sure everything was copied over just fine:
If the output is empty, everything looks good. Otherwise, you probably need to start over.
Everything is ready, unmount everything:
Unplug your USB stick. It's now ready to be used to boot a computer and install Windows on it.