Take the crate off the belt. Look at one, get close enough to reach it, and press SPACE or click. It comes into your hands.

Then put it down a chute. Press 1 to 5, or look at the chute and click.

Mouse to look, A and D to walk the line. You can only carry one crate, and the belt speeds up. Let one reach the end and the line jams. Three jams ends the shift.