The correct slide layout is probably the most significant step that can be taken to ensure that the slideshow will be accessible.
When someone who can see reads a slide, they usually read things, such as text or a picture, in the order the elements appear on the slide. In contrast, a screen reader reads the elements of a slide in the order they were added to the slide, which might be very different from the order in which things appear.
PowerPoint contains a collection of pre-set accessible slide layouts. In order to ensure that the slideshow is maximally accessible, use these built-in templates when constructing the slideshow.