0) One quick point of terminology. Launchpad has the idea of "projects" and "products". A "project" is a collection of "products". For example we have the Bazaar project, which includes the "bzr" program, as well as plugins for "bzr". It is a little foreign to me, since I consider what I work on a "project" rather than a "product". But I understand the need for a higher level grouping, and I can't say that I have better names to distinguish them.
Also, each product gets a set of "series". These are generally meant along the lines of "release series". Most projects will have a development series (by default this is called "trunk"), and possibly some release series. Especially large projects, which will have concurrent development (think of Firefox, which has a 2.0 series, and an 1.5 series, since you get 1.5.1 and 2.0.1).
1) Go to Launchpad itself: https://launchpad.net
2) Go to the products page https://launchpad.net/products
The link on the main page is "register your product".
3) If this is an existing project, you probably want to search and make sure it isn't already registered in Launchpad. In my case, these are always new projects, so I don't worry about it.
4) Follow the "Register a Product" link on the upper left (https://launchpad.net/products/+new).
5) Fill out the basic information for the product. In my case, most of my products fall under the "bazaar" project banner. When creating a new plugin for "bzr", the general convention is to call it "bzr-plugin-name". It certainly isn't required, it is just a convention that I've tried to follow.
6) Change the product to use Malone (Launchpad's bug tracker) as the official bug tracker. This is the link "Define Launchpad Usage" on the left. (https://launchpad.net/PRODUCT/+launchpad). You may also enable Rosetta translations at this time.
7) Change the Maintainer of the product to a shared group. I usually want other people to be able to update the details of the product, update the bug tracker, etc. So I set the project as "owned" by the "bzr" group. That is done by following the "Change Maintainer" link (https://launchpad.net/PRODUCT/+reassign).
8) Now you want to create a Bazaar branch for the mainline of the project. You can do this through the "Register Branch" links. I personally tend to host my branches on Launchpad itself (hosting is free, and it is bandwidth I don't need to pay for). So I do a simple:
bzr push sftp://email@example.com/~bzr/PRODUCT/trunk
A bit of explanation, username must be your launchpad user name, and "~bzr" can be either your username, or the name of the group in step 7. As I mentioned, I prefer the mainline to be a shared branch, so other people can update the mainline if I'm too busy, or cannot be contacted for some reason.
9) Now update the "trunk" series to point to this new branch. There should be a link on the main page (https://launchpad.net/PRODUCT) to the "trunk" series. Or you can link more directly to it at (https://launchpad.net/PRODUCT/trunk).
You want to "Change Series Details" for this series (https://launchpad.net/PRODUCT/trunk/+edit).
10) At this point, you can change the name of the series (maybe you prefer "mainline" over "trunk"). You also can change the description. I usually leave them alone. What I do change is the "Branch". I generally follow the "Choose" link, which lets me search through all branches registered for this product. (Note, pushing to sftp://bazaar.launchpad.net/~USER/PRODUCT/BRANCH-NAME, will automatically register the branch)
11) And you're done. It took a little while, but now you have a fully functioning bug tracker and branch tracker. You are also able to tell people to get your product with:
bzr branch lp:PRODUCT PRODUCT
And they will get the latest development version.
By registering your branches, you now have the ability to link them with bugs, so that users who find a bug, can see that there is already a fix, even if it hasn't been included in mainline yet.
(Edited to fix "Product" versus "Project")