Actually, maybe Dummy is the wrong word as I always think of a dummy as being a finished volume, ready to show people. Obviously a dummy is not a book with the quality of a finished work, though a Blurb could be seen this way, but is a properly edited, sequenced, often printed to the same size as the hopefully published book.
What I was think of is maybe the dummy before the dummy. I always start with small prints of everything I'm thinking could be in the book. Will digitally print out a load of these, (2x3 or 3x3") so I can just work with on a big table to make a sequence. See which go together, which do not, which work as single spreads, others double page spreads and of course, which really do not work at all so are out the edit. Of course, this could be done on screen but I always think it is easier and more intuitive to do it physically. Then when I think I like it, will collate a load of folded A4 sheets of paper to make a blank book. In doing this, have to think if it is all done in one section, ie all the folded sheets together to be bound, or if it will be different sections together, which will then be bound. I will have an idea how many pages it should be by my editing of the small prints. Also taking into consideration title pages, credits, paper to attach the covers etc.
Important to then number each page including endpapers. Then stick in the small prints on the right page, ideally with removable tape. When finished, it will give an idea if the sequence works but also, you will know that if printing yourself on double sided paper, page 4 is next to page 13, for example, and on the other side, page 14 is next to page 3. I made this example up by looking at a nearby newspaper supplement, but you get the idea. Of course, is different if not a folded book as also possible to bind single sheets. I could have photographed some examples, but am away from work for the weekend!