Custom plugin is simply another WordPress plugin that contains your custom code. If you are not a WordPress developer, you won’t be able to create a custom plugin/script.
The alternative option that you could try is to add the custom code to your theme’s
If you really want to create a small plugin, you can check the following articles that has some very basic info on how to create a plugin:
It really requires some WordPress development knowledge, otherwise it may seem confusing to you.