Start by identifying the device name.

myth@ion:~$ lsblk
sdc                       8:0    1  14,9G  0 disk  
├─sdc1                    8:1    1   2,6G  0 part  /media/myth/Ubuntu 20.04.1 LTS amd64
└─sdc2                    8:2    1   3,9M  0 part  

In this case it is /dev/sdc. Proceed to unmount it:

sudo umount /dev/sdc1

Write your ISO file to the drive:

dd if=/home/myth/downloads/ubuntu-20.10-beta.iso of=/dev/sdc bs=1M oflag=dsync status=progress

When not using conv=fdatasync or oflag=dsync, it is wise to call sync after dd exits to ensure kernel buffers are flushed and all data is written to the device. The kernel might do some caching and buffering to do I/O optimization and if you unplug your drive prior to the last blocks being written you might have a bad time.

About the flags

bs=1M replaces the default block size of dd to significantly increase throughput. Going above 4096 does not yield significantly better performance normally, but when using the oflag=dsync setting instead of let's say conv=fdatasync, it has a much greater impact, as data is periodically synced instead of just before dd exits.

status=progress is of course to display the current write speed, elapsed time, and how much data has been written.

oflag=dsync does a write + flush after each segment, which makes progress display the actual throughput.

conv=fdatasync ensures a flush when dd is done, right before it exists, meaning progress might show write figures greatly skewed by kernel buffering.