This is usually a very easy process as there is a shell script at the root of systemrescuecd that does everything. However, I have on a couple of occasions run into a problem where the disk will not boot. The error is

SYSLINUX 3.85 2010-02-20 CBIOS Copyright (c) 1994-2010 H. Peter Anvin et al No DEFAULT or UI configuration directive found! boot:

I recently found on some random forum, which I can no longer locate, instructions using isohybrid. After plugging in the USB stick you do the following (where /dev/sdb is your usb stick)

isohybrid systemrescuecd-x86-2.3.1.iso cat systemrescuecd-x86-2.3.1.iso > /dev/sdb sync

Following that, the box boot into systemrescuecd like a champ.