Flex Slider is an awesome and super basic image slider that you can add to your theme to make it easy for clients to add a slider to their webpages. This slider is well documented and because of that we will jsut be linking to some basic articles on how to achieve this.
First, Download Flex Slider here: http://flexslider.woothemes.com/ (You do not need the paid WordPress plugin)
Next, use Advanced Custom Fields Gallery to manage the images in the slider: https://www.advancedcustomfields.com/resources/gallery/
Last, here is how to PROPERLY call the flex slider code in your theme: https://return-true.com/installing-woothemes-flexslider-into-your-wordpress-theme/