You can start an instant slideshow in Photos on your Mac in a variety of ways, depending on what button is active at the top of the Photos window and what you’re viewing. Here are your options.
1/ Instant slideshows in Photos for Mac — In the Photos view (from the Photos>View menu on your Mac), just put your cursor over a moment or collection, and then click the play button that appears above the thumbnails on the right. When you do, Photos automatically selects all the thumbnails in that moment or collection and opens the Themes panel shown.
Click the theme you want and click Play Slideshow. You can’t trigger an instant slideshow while in Year view, but you can dip into Albums view and play a slideshow based on the All Photos album. (Although there’s no official limit to the number of pictures an instant slideshow can contain, it’s possible that you may have too many pictures for your Mac’s memory to handle, causing Photos or your Mac to crash.)
The Theme panel previews what each theme will look like applied to the first few pictures while the Music panel gives you access to your iTunes library for use as background music.
In Albums view, Control-click (or right-click on your mouse) any album (even those created by Photos) and choose Play Slideshow from the menu that appears.
To play an instant slideshow of multiple albums, select them by Command-clicking (holding down the Command hey as you click) each one, then Control-click one of the selected albums and then choose Play Slideshow. The albums play in the order in which they appear in Albums view. You can also select an album (or several) and then click the play button in the Photos toolbar at the top of the window.
When viewing the contents of an album, make sure you don’t have any thumbnails selected by clicking an empty area between two thumbnails, and then click the play button in Photos’ toolbar. Photos automatically includes all the images inside the current album in your show. (If you did have a thumbnail selected, only that image gets included in the show.)
To play a slideshow of just a few pictures in the album, select their thumbnails first, and click play, Either way, the Themes pane opens; click Play Slideshow to start the show.
To control the order of pictures inside each album, rearrange them in the album before you start the slideshow.
In the Photos Sidebar, Control-click an album and choose Play Slideshow from the shortcut menu. The Sidebar is Photos’ equivalent of iPhoto’s source list. To turn it on, choose View> Show Sidebar.
2/ Slideshow controls — Once the show gets going, you can pause it by pressing the space bar on your keyboard: tap the spacebar again to restart it. To move through the slides manually, use the left and right arrow keys on your keyboard. To end the show, press the Esc key or wiggle your mouse to summon the slideshow controls, and then click the X. Let go of your mouse button and after a few seconds the onscreen controls disappear.
3/ Instant slideshows on your iOS device — You can play instant slideshows in Photos for iOS, too, which is handy when you’re out and about and you want to show off pictures hands-free. To do it, fire up Photos on your iOS device, tap Albums at the bottom of the screen, and then tap an album to open it. Your next move depends on which device you have.
On iPad, tap the word Slideshow at upper right and the show starts playing. The AirPlay icon at upper lets you pick the device the show plays on: the iOS device or, if you’re on a wireless network, an Apple TV-connected monitor. The Options button at lower right summons a pane that lets you control theme, music, whether the show repeats (loops), and how long each slide stays onscreen.
On an iPhone or iPod touch it works slightly differently. Tap an album to open it and then tap Select at upper right. Next, tap or drag your finger across thumbnails to select some photos or tap Select All at upper left. Hold your device in portrait position (in lieu of landscape) and then tap the Share icon at the lower left. In the row of icons that appears at the bottom of your screen, swipe left to scroll horizontally, and then tap the Slideshow icon. Once the show starts to play, tap your screen and you get the AirPort and Options buttons described earlier. (These came from Lesa Snider at Macworld.)
4/ Where are you out recently deleted images in Photos? Apple switched behavior in some release of Photos for OS X: now there’s a Recently Deleted album with a trashcan icon in the sidebar whereas formerly there was a menu item for it in the File menu. If you haven’t deleted anything, the Recently Delete album does not appear.
The Recently Deleted album disappears if you empty it.
5/ How to set a custom viewing order in Photos for Mac — Photos doesn’t allow manually set order in any of its main views, such as the Photos view, All Photos, within moments and so on. You can create a Smart Album (New>New Smart Album) to grab images that meet criteria, which makes it easy to tag an image with a keyword and have it automatically added to that album. But Smart Albums can only be sorted (via the View>Sort menu) alphabetically by title, or chronologically starting with the oldest or newest photo.
To drag images around without having them snap back into a sorted order, you have to create a regular Album (File>New Album) and add images to it. Then you drag as you like. Unfortunately, if you’re regularly working with images you want to add to that album, you have to add them to it by dragging.
More about Smart Albums — This Macworld post tells you a lot more about Smart Albums.