You should get that menu (see graphic) with appropriate device data for your particular Chromebook. Select option 3 to flash a full UEFI capable rom onto your system. Note this is the end of your chromebook effectively once you pass the clearly identified "point of no return".

Note: Some chromebook architectures are unsupported (notably ARM cpu systems). You may get a error message if your system is unsupported.

You can either perform the chromeos rom backup or not as the option presents itself (your choice). You will need that extra USB stick if you choose to backup the original rom. Do you feel lucky punk? Sorry :D

Once it is finished it will prompt you to power off and restart the device. At that point you can insert your new linux boot/install USB stick. From there you simply install like you would on any typical laptop.

I opted to go the full UEFI route on mine but it is optional. Follow the instructions under the FAQs (https://mrchromebox.tech/#faq) The fifth question there is answered with how to proceed to setup your UEFI boot option files correctly.