The simplest way to create a bootable USB stick for Debian installation is to use command dd to copy an ISO image to an empty USB stick.

First, connect an empty USB stick to a working Linux box, and see which device represents the stick:

$ sudo fdisk -l
...
Disk /dev/sdb: 16.0 GB, 16008609792 bytes
255 heads, 63 sectors/track, 1946 cylinders, total 31266816 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0xb745c02d

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1              32    31266815    15633392    c  W95 FAT32 (LBA)

So, on my computer, /dev/sdb is the USB stick. Whatever is on it will be deleted and overwritten in this procedure, so make sure that I use a stick with large enough disk capacity specifically for this purpose.

When I use netinst installer, about 300 MB would be enough. When the access to the network is not guaranteed during the installation (e.g., the network adapter is not properly recognized by the Debian Installer), it is desired to at least use the full CD/DVD media, which would take about 4 GB (just the first medium should be enough for bare minimal installation, after which more full configuration can be resumed).

Download an ISO image for the Debian installer:

$ wget http://cdimage.debian.org/debian-cd/7.4.0/amd64/iso-cd/debian-7.4.0-amd64-netinst.iso

Then copy the ISO image to the USB stick:

$ sudo dd if=debian-7.4.0-amd64-netinst.iso of=/dev/sdb bs=4M
$ sudo sync