Start with Print on Demand
Even if you are not interested in selling your coloring book, starting your research at places such as LuLu.com and Blurb.com is a good idea. You can get an idea of the trim sizes available and what type of paper is best to use. And when you are ready to sell your creation, these print on demand services usually have their own bookstore or a way for you to sell from your own website.LuLu
LuLu suggests using their Book Builder to create your coloring book. For the best results, use the premium white 60# uncoated paper and their Paperback Perfect Bound binding. Their trim sizes include 8.5 x 11 inches, 8.5 x 8.5 inches and 9 x 7 inches.Blurb
Like LuLu, Blurb has their own tools for creating print on demand books. You can use their PDF Uploader, their Book Widget desktop software or InDesign plugin. If you are planning to digitize your hand drawn coloring pages, they suggest using medium or thick lines.
Canva
If you aren't an artist but want to try making a coloring page or an entire book, Canva is a great option. You can start with one of their templates and design your own pages using their large stock of line art elements. It's great for making adult coloring books and books for kids too. When you are ready, you can download your book as a PDF or order a print on demand.
Every place I researched suggests starting with a theme such as holidays, family trips and occasions. Other more general themes include alphabet, mandalas and abstract patterns. Of course, you might also want to include text such as quotes, fun facts and captions in your designs. Another tip was to repeat the best page designs for the front and back covers. You can have them fully or partially colored to suggest what you will find inside.
Custom Coloring Books - LuLu Press Inc
https://coloringbooks.lulu.com
How to Make Your Own Coloring Book - Blurb - RPI Print, Inc.
https://www.blurb.com/blog/how-to-make-your-own-coloring-book/
How to make a coloring book to entertain your kids - Canva
https://www.canva.com/learn/make-coloring-book-easily-entertain-kids/
https://www.youtube.com/watch?v=SrGJW6OmHlU