1 - Add the clips you want to use to the timeline, click and drag with the cursor to select all of them or press Command-A, and then change the "Spatial Conform" setting in the Inspector to "Fill".
2 - It's a good idea at this point to lengthen all of your photos to match the planned length of your slideshow.
With the clips still selected, double click on the time indicator window in the middle of your screen. Type in the new length and press return or enter (typing 2-0-0-0 will make them all 20 seconds).
3 - This is the most important step. One by one, you need to make each clip into it's own "Compound Clip".
Select one clip at a time and either right click or press Option-G to create a new Compound Clip. To give it the default name, you can just press return on your keyboard to save time. We do this to trick Final Cut Pro X into treating that your non-16:9 clip as if it is 16:9.
4 - You're now ready to start adding the Print Stacker effects to the Compound Clips you've created. Stack and stagger your clips in the order you want them to appear in your slideshow.
5 - To adjust the settings of a print, select the clip in the timeline and position the playhead over it. ￼ The settings will then be available on the right side of your screen in the Final Cut Pro X Inspector. Use the controls labeled "Source.Scale" and "Crop.Scale.X" to adjust for portrait oriented or overly cropped prints.
More documentation can be found in the user guide text file that is included with the Print Stacker download.