Installed second kernel from source but getting errors booting up

I have Ubuntu 22.04, originally with kernel 6.5.0-14-generic.

I followed this guide:

https://phoenixnap.com/kb/build-linux-kernel

to compile and install the latest kernel sources. However, now when I boot up I get this menu:

UbuntuAdvanced Options for UbuntuUEFI Firmware settings

If I choose Ubuntu it says:

error: bad shim signatureerror: you need to load the kernel firstPress any key to continue

If I choose Advanced Options for Ubuntu I get:

Ubuntu, with Linux 6.8.0-rc1+Ubuntu, with Linux 6.8.0-rc1+ (recovery mode)Ubuntu, with Linux 6.5.0-14-genericUbuntu, with Linux 6.5.0-14-generic (recovery mode)

If I choose 'Ubuntu, with Linux 6.8.0-rc1+' I get:

Loading Ubuntu, with Linux 6.8.0-rc1+...error: bad shim signatureLoading initial ramdiskerror: you need to load the kernel first

If I chose 'Ubuntu, with Linux 6.5.0-14-generic' it works.

When I run find /boot/vmli* I get:

/boot/vmlinuz/boot/vmlinuz-6.5.14-generic/boot/vmlinuz-6.8.0-rc1+/boot/vmlinuz.old

Where did I go wrong? I just want to understand why this hasn't worked as planned. It feels corrupted.

I'd like to understand how to install different kernels and be able to boot from them seamlessly.