Boot a GNU/Linux installer on USB

How to boot from USB

Connect the USB drive. Check dmesg:
$ dmesg

Check that it wasn't automatically mounted. If it was, unmount it. For example:
$ sudo umount /dev/sdb1
# umount /dev/sdb1

dmesg told you what device it is. Overwrite the drive, writing your distro ISO to it with dd. For example:
$ sudo dd if=gnulinux.iso of=/dev/sdb bs=8M; sync
# dd if=gnulinux.iso of=/dev/sdb bs=8M; sync

Boot it in GRUB using the "Parse ISOLINUX config (USB)" option (it's in default libreboot grub.cfg, at least).

If the ISOLINUX parser won't work, then press C to get to GRUB command line.
ls
Get the device from above output, eg (usb0). Example:
cat (usb0)/isolinux/isolinux.cfg
Either this will show the ISOLINUX menuentries for that ISO, or link to other .cfg files, for example /isolinux/foo.cfg.
If it did that, then you do:
cat (usb0)/isolinux/foo.cfg
And so on, until you find the correct menuentries for ISOLINUX.

Now look at the ISOLINUX menuentry. It'll look like:

	  kernel /path/to/kernel
	  append PARAMETERS initrd=/path/to/initrd MAYBE_MORE_PARAMETERS

GRUB works the same way, but in it's own way. Example GRUB commands:
linux (usb0)/path/to/kernel PARAMETERS MAYBE_MORE_PARAMETERS
initrd (usb0)/path/to/initrd
boot
Of course this will vary from distro to distro. If you did all that correctly, it should now be booting the ISO the way you specified.