arrange the photos for slide show

Put them in an album and sort the album alphanumerically.

Then Control (right) - click on the album in the sidebar and select Create ➙ Sliceshow ➙ Photos from the contextual menu.

Note: only the Classic and Ken Burns themes will maintain the order as you imported. The other themes use more than one photo per slide and will pick out the images that best fit the layout of the slide.

This user tip also describes the process: How to turn an Album into a Slideshow and preserve the Sorting, 2019 Version