Correct me if I'm wrong, but...

When I use "batch script" (like @ECHO OFF and ECHO.) for my RunOnceEx.cmd rather then the kind of script used here, then the only place where I should list RunOnceEx.cmd is in WINNT.SIF like this right?

[GuiRunOnce]
%systemdrive%\Install\RunOnceEx.cmd

Or does RunOnceEx.cmd start autmatically like WINNT.SIF?

That way you don't have to list it anywere

Could someone please give me a correct answer for this or else I can't make my own CD

As I understand it correctly you have to leave the [GuiRunOnce] empty, create a cmdlines.txt file as described at the link you posted, by creating that file RunOnceEx.cmd will automatically be launched, see my post here 'Can anyone chck my codes' for an example.