-a bootable USB-Stick
First execute YUMI and click I Agree.
Then you can select your USB-Stick. Be sure you select the right one.
If your USB-Stick isn't formated in the FAT32 filesystem click format.
Next step select your image of the your Distribution.
If you don't have a image you can also download it by clicking "Download the ISO"
Now you can click "Create" and the Files get written to your USB-Stick.
The only thing you have to do anymore is to enable the USB-Boot in the BIOS.