Which you can't if it is directly overhead.
Try it out to sea, bro.
https://earthsky.org/favorite-star-patterns/how-to-use-southern-cross-to-find-south-celestial-pole
Another slighty trickier, but more accurate, way of finding south is to use the Southern Cross and the pointer stars from the neighbouring constellation, Centaurus. Draw a line through the two stars at the ‘head’ and the ‘foot’ of the cross and extend it to the dark patch of the sky the same way as in the first method (Line 1). Then join a line between the two pointers (Line 2). Find the middle of Line 2 then draw a perpendicular line down toward Line 1 until the lines meet. The point at which the lines 1 and 3 intersect is the south celestial pole. From there just look straight down to the horizon and you’ve found south. Illustration and caption via ABCScience.