Instructions
Press "I" on your keyboard to open your inventory.
Right-click the item you want to split. When right-clicked, the item splits in half. For example, right-clicking on a stack of 20 sand blocks splits the sand into two stacks of 10 sand blocks apiece.
Left-click the inventory stack to hold it. Right-click any open spot in your inventory to place just one item there. For example, left-clicking a stack of 20 sand blocks and right-clicking an empty spot in your inventory places only one sand block there, allowing you to split your stack one at a time.