This guide will show you how to install a bootable linux livecd image to USB stick.
Get the livecd image
First you need to choose livecd distribution. SystemRescueCd is good choice because it supports 32bit and 64bit processors on the same image. Its also based on gentoo.
Download the image of your choise:
$ wget http://example.com/linux-livecd.iso
Convert livecd image to hybrid mode
Most modern LiveCDs are already in hybrid mode. If your LiveUSB doesn't boot without this step then you have to convert it to hybrid mode. Hybrid image means image that can be booted from either CD-ROM or USB drive.
# emerge syslinux $ isohybrid linux-livecd.iso
Write livecd image to the usb drive
Now you can write the hybrid image to usb drive. Be sure to select the device node /dev/sdX, not partition node /dev/sdX1.
# dd if=linux-livecd.iso of=/dev/sdb bs=4M
Now you can reboot. You must select the usb drive as startup device in bios.