[ubuntu] Installing a Macbook Pro 2019 with ubuntu-21.10-desktop-amd64

I'm attempting to perform the install (to an external USB-C disk) described in the title.

It seems no matter what I try, I get a black screen shortly after selecting my external disk to boot from.

I tried copying the .iso to my little USB flash drive using dd on macOS or cp on Linux. I also tried converting the .iso to .img using:
...but I still get a black screen when I try to boot from it. BTW, I got a .img.dmg file, not a .img file, and dd'd that.

What do I need to do to initiate the install process?

Thanks!