How do I use a collection of photos in LrC to make a book using Mimeo.

Using a collection is a good idea.

Once all the selected photos are in the collection, put the photos (using drag & drop) in the order you them to appear in the book. Verify if your order is coherent with the book you want to create by moving thru all the photos in view mode. This somehow "simulate" the reading og th photo book. Once satisfied with the order, export all the photos of collection using a reneme preset to add a counter infront of the file name.

This will make your jpeg's appear in the file exporer in the same order as in the collection, which will make creating th book much easier and faster.

I am a Blurb user (~15 books). A quick look online suggests to me that Mimeo simply requires you to upload your photos to the Photo Book creator interface. (unless you can tell me more about options)

In that case you would simply [Export] your edited files from Lightroom-Classic to a folder in your computer. From there you will upload them to the Mimeo interface.

Do not import these (newly exported) files to Mac Photos- that will just add complications.

If you want to involve the Lr-Classic Book module, you can design your book in the Book module then [Save] your book pages as JPG files. These JPGs are then uploaded to Mimeo. A problem I see with this is that the Blurb Book Page Sizes and ratios may not match with the Mimeo Page Sizes- meaning the JPGs from a Blurb File export will not eactly 'fit' a full-page in Mimeo.

I believe (my simplified view) your workflow might be-

-In LrC assemble the photos you want for your Mimeo book in a standard Collection.

-Edit the photos in the Develop Module to your (color) liking.

-Select all the photos in the Collection and Export them to a local folder. (Use Size and ppi suitable for book quality).

-Upload the photos to Mimeo......

I am a Windows user so I don't know anythinng about Photos on the Mac. But if you have edited the images in Lightroom, and have them looking the way you want them to look then it seems to me that all you need to do is export them to a folder on your desktop. Now, whatever you do with that folder of images from that point on would be whatever process you normally use to create your photo books. You will need to export the images from LrC. Otherwise the images will not have the adjustments that you made, And may not be in the appropriate format needed for creating the photo book.

I have never used that service. I haven't produced a photo book for several years. The last time I did produce one I used a company that doesn't interface with LrC. The process I used was to export JPEG images to a folder on my computer and then use the software provided by the company to create the photo book. I suspect that will be the workflow you will need to use in this instance.