RPi - changing boot logo and splash screen

I want to remove boot messages, boot logo and add a custom splash screen.

I found 2 tutorials:
https://scribles.net/customizing-boot-up-screen-on-raspberry-pi/
https://yingtongli.me/blog/2016/12/21/splash.html

First the text on top of the /boot/config.txt is confusing:

#Please ensure you edit from the DietPi-RamDisk location /DietPi/*

Does that mean i have to edit /boot/dietpi/dietpi-ramdisk?

Then adding for example ‘logo.nologo’ to /boot/cmdline.txt has no effect.