Booting from USB is easy on a Pi1, Pi2, or Pi3. This can be useful in mamy cases to speed up the performance of your pi when using a slow sd card. It can also provide a reliable location where you operating system data resides. Sd cards are prone to data corruption from writes a operating system ussually does.
Write the latest Raspbian image to an SD card and boot it.
Write the latest Raspbian image to a USB drive and plug it into the Pi.
Run blkid to determine the PARTUUID of the USB drive.
Edit /boot/cmdline.txt and change root=PARTUUID=xxxxxxxx to match the PARTUUID of the USB drive.
Reboot. The Pi should be running from the USB drive.
If you ever need to boot from a different USB drive or from the SD card, it’s necessary to FIRST mount the boot partition of the SD card (/dev/mmcblk0p1) and edit the boot partition’s cmdline.txt so that root=PARTUUID=xxxxxxxx matches the device you wish to boot from.