Hi,
I assume that you have created your slideshow in the photos app. If so, follow these steps:
- Open the slideshow in Photos.
- Click on the Export button in the upper right hand side of your screen.
- Choose desktop as the destination.
- Click Save. (It will save to your desktop as an m4v movie that works in iMovie.)
- Open iMovie
- Open a project in iMovie (Click on Projects button at top of screen and then click on the gray icon with the + sign and select to create a movie).
- Drag your slideshow from your desktop to the timeline of the iMovie project you just created.
- Add your music to the audio timeline just below the video time line.
- Position your audio clips where you want them.
- Trim and edit your audio clips as you want.
- Share out as a File to your desktop.
Here's a link to an online help menu with a good search feature that will explain in more detail, if you need it, the different steps involved in the above procedures.
https://help.apple.com/imovie/mac/10.1/
-- Rich