There are a lot of ways how to make a bootable usb , but i will show you a very simple and effective method
Best way to do this is by using a little but very good software,his name is Rufus, rufus is a free softawre and you can find it to official website
and is a very small software only 1mb. First you have to downlaod os example linux,and then go to google and download rufus. After that put an usb drive on your pc.
Open rufus and select your is OS iso file.
CAUTION: All your usb device data will deleted you first you have to check if you have sommething
important in you usb device, after that you are ready to start makin bootable usb device.
After select OS iso file, click start and you will show a dialog, Caution all data on removable
device will be deleted, Are you sure do you want to continue, after that click ok and, wait until
process is finished. This process can take you about 3 to 15 minutes, this it depends on witch
Operatin system are you puting on usb device. After process is finished, remove usb device from pc
and close rufus program. Now you remvable device is ready to start booting.
When process is finished you will see Done! on rufus program bar.
There are a lot of other ways how to make a bootable usb device, but I think this a very simple
way and is very easy to follow by anyone, who want to make a bootable device in a very simple way
Rufus is a really good choise to use