How to make an Item with a (Consistent) Inventory?

So, in my mod, I made an item called a Pouch. This is an item with five inventory slots that can be carried around with you...

However...

The GUI for the Pouch works, the inventory pops up, you can put things in it...

But as soon as you close the pouch and open it again, the items are gone...

Help?

*I was going to put in pictures, but you cannot insert pictures from computer... Why?*