How do I make a USB stick bootable?

How do I make a USB stick bootable?

You should be able to image your USB Thumb drive with the official imager and just plug it in and go, I think so anyway?
Raspberry Pi OS – Raspberry Pi

It’s a bit old but might help
Running Raspbian from USB Devices : Made Easy - Raspberry Pi Forums

i have this bookmarked for when i’m ready to give it a try .

To making a bootable USB drive of OS. Download the EtcherPC and the OS file and after that make bootable Drive. Now, Power off your Raspberry Pi and insert the SD card, After that wait for 10 seconds and then power on the Raspberry Pi to install the OS.

If you have a just bought a Pi4B or Pi400 it should be already able to boot from USB without having to do anything fancy. All you need to do is image your USB drive with the official image utility.

USB mass storage boot - Raspberry Pi Documentation

My Pi400 booted up from my USB thumb drive no problem after imaging it with the Pi imager.
My Pi 4B also booted up no problem from that same USB thumb drive.