That's a good starting point.
So you've looked at the existing code for the tool_uploaduser (and possibly tool_uploadcourse as well, as that does something similar) and, presumably, you can see the basic flow of the page that shows the upload form and the way it processes the file. But, you've stated there are "some things that I do not understand". Which bits do you not understand? I guess someone could go line by line through each of the files in tool_uploaduser and explain exactly what each line does, but that's probably not a good use of anyone's time (do you need someone to explain what "require_once()" does, do you need an explanation of "optional_param()", is it "admin_externalpage_setup()" that you are getting stuck on, or is it something else in that plugin?)
So you've looked at the existing code for the tool_uploaduser (and possibly tool_uploadcourse as well, as that does something similar) and, presumably, you can see the basic flow of the page that shows the upload form and the way it processes the file. But, you've stated there are "some things that I do not understand". Which bits do you not understand? I guess someone could go line by line through each of the files in tool_uploaduser and explain exactly what each line does, but that's probably not a good use of anyone's time (do you need someone to explain what "require_once()" does, do you need an explanation of "optional_param()", is it "admin_externalpage_setup()" that you are getting stuck on, or is it something else in that plugin?)