First of all you need to download Rufus, you can do it from the following website: https://rufus.akeo.ie/
So in a list it’d look like this:
1) Go to the official rufus website.
2) Download and open rufus portable.
3) Select your preferred language.
4) Select the device you want the operative system installed onto (look for a drive the size of the USB stick you plugged in)
5) Select create bootable disk and from the drop down list select ISO (on the picture “FreeDOS” is selected), and then click on the disk on the right to look it up on your computer. Once located;
6) Hit start and wait for it to finish.
Then reboot the computer and have it boot from USB and you’re all set.