how can I start playing a slide show in photos?

Hi,

there are couple ways to quickly play a slideshow:

• select photos and right click -> Play Slideshow
• open any album and click Slideshow on the top -> click Play Slideshow button

Or you can create and save a slideshow project.

There is the link with basic information about slideshow:

https://support.apple.com/en-ca/guide/photos/phtae8c6d40/7.0/mac/12.0

One more thought - Which system version are you using? Do you want to use video clips or Live Photos in your slideshow?

Slideshows are behaving buggy since macOS 10.15 Catalina - videos are not playing the full length, and Live Photos and animated gifs are only shown as still frames. If you are having animated items in your album, you may want to view it as a memory, rather than a slideshow. When viewed a memory, at least some of the videos and Live Photos will be animated. Then try the button "Show as Memory" on top of the album instead of "slideshow".