Help Making a Custom Plugin for Text Widget

I used generatewp.com to make a text widget that I would like to have as a plugin for my multisite installation. I have the code and when I put it within my plugin, I see the plugin to activate, but when I activate it, there is not any widgets in Appearance > Widgets. I just copied/pasted the widget code into my functions.php and I had the widget. I put in my sidebar and saved, but it did not stay there. When I went back to my site I didn’t see it, and on Appearances > Widgets the widget was not in my sidebar.

Looking for some good how to resources beyond the codex for making this plugin. Thanks!