How the backpack looks once opened.
Start out with a chest, eight leather and a cow.
The various crafting recipes, hold the item stack on the left in your main hand, and the item stack in the middle in your off hand then right click a cow to craft the result (Item stack on the right)
A backpack full of building materials, contains a chest link, and a hotbar link for hotbar slot 1.
After right clicking the cow, you will have a backpack.