Once the project is presentable — not perfect, just presentable — you're ready to announce it to the world.
This is a simpler process than you might expect. First, set up the announcement pages at your project's home site, as described in the section called “Announcing Releases and Other Major Events”). Then, post announcements in the appropriate forums. There are two kinds of forums: generic forums that display many kinds of new project announcements, and topic-specific forums where your project would be welcome news.
Make sure the announcement includes key words and phrases that will help people find your project in search engines. A good test is that if someone does a search for "open source foo bar baz", and your project is a credible offering for foo, bar, and baz, then it should be on the first page of results. (Unless you have a lot of open source competitors — but you don't, because you read the section called “But First, Look Around”, right?)
As of early 2022, the best general forum for announcements is probably https://news.ycombinator.com/. While you are welcome to submit your project there, note that it will have to successfully climb the word-of-mouth / upvote tree to get featured on the front page. The subreddit forums related to https://www.reddit.com/r/opensource/, https://www.reddit.com/r/programming/, and https://www.reddit.com/r/software/ work in a similar way. While it's good news for your project if you can get mentioned in a place like that, I hesitate to contribute to the marketing arms race by suggesting any concrete steps to accomplish this. Use your judgement and try not to spam.
You might also consider submitting an entry for your project at the FSF's Free Software Directory https://directory.fsf.org/, though that is more about helping its long-term findability rather than about soliciting attention at the moment of launch.
Topic-specific forums are probably where you'll get the
most interest, of course. Think of discussion forums where an
announcement of your project would be on-topic and of
interest — you might already be a member of some of
them — and post there. Be careful to make exactly
one post per forum, and to direct people to your
project's own discussion areas for follow-up discussion (when posting
by email, you can do this by setting the
Reply-to header). Your announcement should
be short and get right to the point, and the Subject line should make
it clear that it is an announcement of a new project:
To: email@example.com Subject: [ANNOUNCE] Scanley, a new open source full-text indexer. Reply-to: firstname.lastname@example.org This is a one-time post to announce the creation of the Scanley project, an open source full-text indexer and search engine with a rich API, for use by programmers in providing search services for large collections of text files. Scanley already has running code, is under active development, and is looking for both developers and testers. Home page: http://www.scanley.org/ Features: - Searches plain text, HTML, and XML - Word or phrase searching - (planned) Fuzzy matching - (planned) Incremental updating of indexes - (planned) Indexing of remote web sites - (planned) Long-distance mind-reading Requirements: - Python 3.9 or higher - SQLite 3.34 or higher For more information, please come find us at scanley.org! Thank you, -J. Random
(See the section called “Publicity” for advice on announcing subsequent releases and other project events.)
There is an ongoing debate in the free software world about whether it is necessary to begin with running code, or whether a project can benefit from being announced even during the design/discussion stage. I used to think starting with running code was crucial, that it was what separated successful projects from toys, and that serious developers would only be attracted to software that already does something concrete.
This turned out not to be the case. In the Subversion project, we started with a design document, a core of interested and well-connected developers, a lot of fanfare, and no running code at all. To my complete surprise, the project acquired active participants right from the beginning, and by the time we did have something running, there were quite a few developers already deeply involved. Subversion is not the only example; the Mozilla project was also launched without running code, and is now a successful and popular web browser.
On the evidence of this and other examples, I have to back away from the assertion that running code is absolutely necessary for launching a project. Running code is still the best foundation for success, and a good rule of thumb would be to wait until you have it before announcing your project. However, there may be circumstances where announcing earlier makes sense. I do think that at least a well-developed design document, or else some sort of code framework, is necessary — of course it may be revised based on public feedback, but there has to be something concrete, something more tangible than just good intentions, for people to sink their teeth into.
Whenever you announce, don't expect a horde of participants to join the project immediately afterward. Usually, the result of announcing is that you get a few casual inquiries, a few more people join your mailing lists, and aside from that, everything continues pretty much as before. But over time, you will notice a gradual increase in participation from both new code contributors and users.
Announcement is merely the planting of a seed. It can take a long time for the news to spread. If the project consistently rewards those who get involved, the news will spread, though, because people want to share when they've found something good. If all goes well, the dynamics of exponential communications networks will slowly transform the project into a complex community, where you don't necessarily know everyone's name and can no longer follow every single conversation. The next chapters are about working in that environment.
 Note that announcing your project usually comes long after you have open sourced the code. My advice to consider carefully the timing of your announcement should not be taken as advice to delay open sourcing the code — ideally, your project should be open source and publicly visible from the very first moment of its existence, and this is entirely independent of when you announce it. See the section called “Be Open From Day One” for more.