Modifying custom pages

Hi,

I created a custom page as per the instructions on codex. It shows up when I click view site so I know it is working. (There are other custom page templates inbuilt with the theme…)

However, I am confused as to how to modify it. I thought of editing the style.css but then realised that doing that will change the original theme template as well.

Can anyone shed some light on what I should do? I have no idea how to write php but have been editing themes from the style.css files. (I would like to modify my custom page as well as those inbuilt ones…so please explain how I find those too.)

Please talk to me like I am 12 years old and not go full throttle with codes.

Thanks in advance…