Bootable usb is very important to install any os.
You make bootable usb by following the steps given below.
1.You need pendrive.
2.You need the iso file of os which you want to install.
3.Then install etcher software to make bootable pendrive from this link https://www.balena.io/etcher/
4.After that install the software.
5.Then open the etcher software and click of your usb option and choose the usb.
6.After that import the iso image file.
7.At last click on start.
How to make bootable usb. 