OPDS stands for Open Publication Distribution System. The 1.0 protocol is based on the ATOM syndication standard and 2.0 is based on JSON LD. OPDS is used to communicate via the world wide web a catalog of books and their related metadata resources. The goal of OPDS Catalogs are to make Publications both discoverable and straightforward to acquire on a range of devices and platforms. To support that goal, this specification strives to provide a framework for describing how a Publication may be acquired while not attempting to constrain this very complex topic. Commonly used acquisition scenarios may be specified in an update to this specification.
An OPDS server is simply an application server or a file server that hosts and publishes an XML or JSON document that conforms to the OPDS protocol. Client applications like RSS readers or the Palace applications interpret these documents to display catalogs of content, transact with resource providers and download books. This creates a highly scalable highly interoperable ecosystem from content distribution.
You can learn more here about OPDS
The basic answer is: "set up an OPDS server that gives access to your catalog". But you'll need to answer two questions to figure out what features the OPDS document server needs to have and what design you should use.
If you're the author of a book, or ebook service provider and you want to get books into Palace, you should know that The Palace Project server is not a bookstore or a library. It's a mobile and web back-end interface to a large number of libraries. There are no books in "Palace" per se; all the books are in one library or another managed by a content service or content repository. To get your books into Palace, you'll need to get it into one of those systems. At that point it will be available to libraries to add to their collection and make available to patrons of that library.
If you are an individual author or a publisher wanting to have your books hosted and distributed through Palace, there are several organizations that participate in this ecosystem that can work with you individually:
The rest of this document assumes that "you", the person who has books they want to get into Palace, represent a publisher, distributor, university, public library, or similar institution. In other words, we assume you represent an organization that deals with hundreds or thousands of titles--so many titles that it makes sense to set up your own OPDS server.
The first thing a user sees when they open the Palace App is the library registry, a searchable list of OPDS servers that the application tries to match a person to. If you want individual people to be able to find your OPDS server, then you need an entry in the library registry. If your goal is to distribute books through libraries or similar institutions, then you don't need an entry in the library registry. Some organizations that could/will/do show up in the library registry: