If you want to use a PPA, then there is no way to upload your .deb files directly. What you have to do is to upload your source package files and launchpad will take care of uploading the binary package files in a controlled environment for security reasons. This also has the advantage that binary packages for all supported architectures will be generated.
If you want just to make a .deb file available in launchpad without using a PPA and you already have a project, you need to create a milestone, work on the bugs you want to fix and create a release when you're done. Once the release is created, you can upload all the files you need and they will available in the downloads section of your project.
