About Fly-Pie: This is only a Gnome shell extension (i.e. for part of Linux). It can't be used on Windows.

In order to write a plugin, you need to create a plugin.ini file (that contains some general info about the plugin) and a main.lua file (that contains the Lua code). See https://xournalpp.github.io/guide/plugins/plugins/#plugin-structure
For the main.lua file you can basically just copy the Lua code you saw in #4658
For the plugin.ini file you can copy this file and modify the author name and description.

On Windows the plugin.ini and main.lua file can be put either into C:\Programs\Xournal++\share\xournalpp\plugins\PLUGINNAME\, where you replace PLUGINNAME by the name you want to give to the plugin.
Alternatively (recommended) you can also put them into C:\Users\<user>\AppData\Local\xournalpp\plugins\PLUGINNAME, where <user> is your user name. You may need to create the plugins subfolder of xournalpp in that case.

Writing a plugin to add "sticky notes" to a Xournal++ document is a bit tricky, since all that makes a Xournal++ document are the background, strokes, images, latex images and text. Your plugin could create some filled strokes looking like the shapes above (and some accompanying text if you wish). Would that suit your needs?