Download Ubuntu ISO file
https://ubuntu.com/download?_ga=2.198192559.122687641.1624992911-1189331534.1624992911

Download and install Rufus (A USB writing tool)
https://rufus.ie/en/

Launch Rufus

Select USB stick

Boot selection: FreeDOS

Click 'Select' to the right of 'Boot selection' to select your downloaded
Ubuntu ISO file.
Note: The 'Volume label' will be updated automatically

Click 'START'

Note: Rufus might require additional files to complete writing the ISO
If Rufus requires those files it will prompt you for permission to download them

Note: Rufus might notify you that you can choose to write to the USB stick
in an ISO format or in a DD (Disk image) format. I recommend ISO since that
means you can also use the USB stick as a regular USB stick even with the OS on it.

Note: All of the data on the USB stick will be destroyed during this process.
Make sure there is nothing on there that you don't want destroyed.

When Rufus has finished writing to the USB stick the status bar will be green
and the word 'READY' will be written inside the status bar.
Click  'CLOSE' to finish the write process.

Here is a resource for learning to boot from USB:
https://www.acronis.com/en-us/articles/usb-boot/