How do I add a slideshow with music?

Your photos one by one, full screen, crossfading to music from the library or your own, from the moment of the track you choose.

Switch it on and choose the music

Open the gallery's Design tab and go to Slideshow. It's on in every gallery to begin with; switch Slideshow off to take it out of the gallery.
Under Music, press Choose music, or Change once a track is chosen.
Press ▶ beside a track to listen, and pick the one you like: it's saved at once. No music plays the photos in silence.
Suggested for this gallery lists the library's tracks for the gallery's type, the Type in Settings › Details, such as Wedding or Family. The rest of the library is under More from the library. Press Done.

Under Where the music starts, the track's waveform has a window on it, as long as the slideshow. Drag it to the part that should play first, or click elsewhere on the track to move it there. Once you've clicked it, the arrow keys move it a second at a time.
Press Play from 0:42 (it says where the window is) to hear that part. It stops where the window ends.
At the end of the track, the music starts again from the same moment, not from the beginning.

In the music window, under Your music, press Upload a track and choose an MP3 or M4A file of up to 30 MB. Once it's in, it's chosen for the gallery.
Your tracks are kept for every gallery of the brand. A track's ⋯ menu has Rename… and Remove….

Seconds per photo: slide from 3 to 8. Every gallery starts at 5.
Photos: The whole gallery, or Chosen sections with the ones to play ticked. The line under them says how many photos that is and how long it plays.

Slideshow at the top of the gallery plays it full screen, from the first photo.
Each chapter the slideshow plays has Play under its title, for that chapter alone, when the gallery shows chapter openings.
Copy the slideshow link copies the gallery's link with #slideshow at the end. It opens on the slideshow's own first page, where your client presses Play.
Photos crossfade with a slow zoom; for people whose phone or computer asks for less motion, they only crossfade. The controls fade away while it plays, and a tap or a move of the mouse brings them back: back, pause, next, sound and close.
On a phone, a swipe goes to the next or the last photo, and a swipe down closes it. On a computer, Space pauses, the arrows go back and forward, and Esc closes it. The screen stays on while it plays.

Good to know

Upload only music you have the rights to use in your galleries, such as a track you've licensed for it or made yourself.
Films and hidden photos are never in the slideshow.
Your tracks belong to this brand. Your other brands don't see them, and nor does anyone else. Library tracks are the same for every photographer, and take none of your storage; your own take a little, like photos.
A new gallery starts with a library track that suits its type, once the library has one. You can still pick any track, or none.
The slideshow is part of the design: Save as preset… keeps it for new galleries, with its music, where it starts and the pace. Chosen sections stay with the gallery they were chosen in.
Phones and browsers play sound only after a tap or a click, so the music starts when your client presses Slideshow or Play.

There's no Slideshow button in the gallery: Check that Slideshow is switched on in Design › Slideshow, and that the gallery has at least two photos that aren't films or hidden.
The slideshow plays without music: Check the sound button at the bottom of the slideshow and the phone's volume. If the track has been removed since you chose it, choose another in Design › Slideshow.
A track won't upload: Tracks can be MP3 or M4A files of up to 30 MB. Export it again as an MP3 and try once more.