The basics of accessibility covered in Microsoft Word also apply in PowerPoint. The main difference is the read order of the slide (see Reading Order below), since PPTs do not use headings the same was as Word. The following best practices are provided to help you maximize the accessibility of your PowerPoint presentations.
Avoid adding any text boxes or adding items on top of the content boxes already provided in the layout templates. The reason is because screen readers may jump over/ignore items, like text boxes, that are added to the pages and exist outside of the content boxes provided.
Use the Selection Pane option under the Format ribbon (or the Arrange menu from the Home ribbon). The order in which items are read will be listed from bottom to top - the first item read will be at the bottom. You can use this tool to reorder the read order, if needed. Remember, the title should always be read first.
The title of each slide should be unique. This allows someone to know what information is on each slide. If the title is the same between slides, then it is unclear how/why the content of the slides has been broken into multiple slides. If the info does span multiple slides, include "continued" in the title to indicate that the info is spilling over from the previous slide.
Do not use shapes. Often shapes are used behind text to create emphasis. Most screen readers will not identify shapes, and you will often split up your text in awkward ways to make it fit withing the shape.
Any transitions you add to your slides should be done using the "on click" option, versus timing the animations and transitions. This allows the user/viewer to control the speed at which they view the content and progress through the slides. You can adjust this setting from both the Transitions and Animations ribbons. If you must use transitions, keep them simple and untimed. Try not to use fancy animations. Doing so can be very distracting from the central message, and in some cases, too much flashing or movement can induce seizures.