Left click a circle, square, or triangle not in the inventory to add it to the inventory. If the inventory is full this will do nothing. Circles have a max stack of 1, squares have 5, and triangles have 10. Left click an item in the inventory to select the full stack, right click to grab one at a time. If you have an item grabbed, clicking on a slot that is empty or has the same item and is not full will add it to that slot. Clicking on a slot that has a different item will swap the items. Middle clicking while holding an item will delete it.