Drag to rotate it, on desktop or on touch. Rotating is essential rather than optional, because blocks you need are frequently on a face that is currently turned away from you.

Only blocks that are currently reachable will glow. Click or tap a glowing block to send it toward a matching crate.

You jam and the level ends. The way to avoid it is to finish crates rather than start several at once — completing an order ships it and returns the slots, while holding singles from many orders leaves you with nowhere to put anything.

Earlier than feels necessary. A slot opened while you still have options gives you room to manoeuvre, whereas one opened as a last resort tends to postpone the same jam by a single move.