As I understand it, to boot from usb on the 3B+ you simply create an image on your usb media, plug it into the Pi, then turn on the Pi.
Unfortunately this isn't work for me.
If for example, I take my SD card out and put it into a usb adapter, and try to boot from it, I first get the rainbow screen, then the grey screen with the Pi logo. After that the Pi cuts signal briefly to the display, then when it comes back it's a black screen. This is when booting from a good SD card that will boot perfectly find in the SD card slot.
Next I installed an image on a USB SSD. When I plug that into the Pi and try to boot, I get no monitor output at all. The Pi turns on, then a few seconds later I see the light on the SSD come on to show that it has power/is connected, and that is it. Nothing else happens. There's no signal to the monitor and no evidence that Pi is doing anything.
Am I missing something?