This project is abandoned and its default file will likely not work with the most recent version of Minecraft. Whether this project is out of date or its author has marked it as abandoned, this project is no longer maintained.
Put the BagInventory.jar file in your plugins folder.
For your basic understanding of the plugin you can op your self and run /bag (Read below)
After running /bag if your oped, BagInventory would have auto disabled.
Run /bagtoggle to re enable it.
Place your "Bag" in the appropriate slot
For purposes of making it in a shop or a custom mob drop the code behind the Bag checks for a meta data (Custom Stuff)
So, the bag is titled Bag but is Bold and Gold colored.(Coding Purposes)
Other wise just add them to chests manually and players will find them.