How it works?
Say you have windows, then first download it from here. After that install the utility on Windows. Plug in your Pen drive and Format it to "Fat32".
Then double click on the desktop icon and start the application which should bring you something similar to below.
Now the "Distribution" section is for those of you who want to use UNetbootin to download the Linux ISO files automatically. Since you've already got the ISO file, you can skip that and move to the below section in which you can show where the downloaded .ISO file is located.
Then choose your Pen drive path from the "Type" section, then click "OK", and it would automatically install the content to your USB drive and when done, reboot your PC and boot into your Linux distro via the USB (make sure to set the proper boot sequence in BIOS).