I don't know about any guide I can point to, but you can start over on the USB stick by recreating the partition table (msdos) and partition (fat32). I recommend using gparted for things like that if you are unfamiliar with the process. This ensures the USB is correctly setup to boot from.
the USB might not have had a partitiin table to begin with. it's not required for them. the file system can be directly on it and often is. however, when you made it into a bootable using an image, it would create a table if the image requires it.