Connecting two tables

So I am having an issue figuring something out.

So, I have orders and inventory items.

I want to be able to add an inventory item to an order. I have that part down by going to the inventory item and editing the record and choosing to only select the orders available.

But what I am having an issue with is getting the order to show the inventory item and im not sure how to connect them together at the time of adding it to the order? Does that make sense?