I have seen all sorts of setups to glue a scarf joint but none as good as the traditional method. All you need is two squared edges, one clamp and some scrap blocks of the right size. Obviously preparation of the joint is crucial and your stock should be of even thickness, even if the head and neck are not the same thickness as you see in the photo. The hide glue’s fast grab and the parallel clamping surfaces mean that the wood has no reason to move. DON’T try this with any other glue! But then why would you want to use any other glue on a guitar?