How to add 6 collections together to make 1 Slideshow in Lightroom

@brucecoull   This might be pertinent to your question:

An example-         Might be for a book, and images are already organized in multiple Custom sorted collections (as Chapters). And then to get all the Collections combined into one, and have the photos stay in the exact order of the originating Collections? 

Method-

1. Create a new (empty) Collection (call it "New Collection" or "Slideshow")
2. Select the "New Collection" (the screen will be blank, no photos)
3. ***Immediately set the New Collection Sort Order to "Custom" (even though it does not contain photos)
4. Now select one of your 'pre-sorted' collections (eg Chapter-1),
5. [Ctrl+A] to select all photos, and drag&drop the photos onto the "New Collection"
6. Repeat Steps-4&5.  for each or your sorted collections in the order you want (Chapters-2, -3, -4, etc)

 (Do not then change the sort method on the Tool Bar!)

Make one new collection that contains all the images. Apart from sort orders like 'file name' or 'capture date', Lightroom also offers a custom sort order where you can drag images into any order you want.