The self-signed certificate is the simplest kind of certificate to
acquire, since it doesn't require working with any certificate
agencies. You can generate a self-signed certificate on your own, and
you can use it to sign your p3d files.
A self-signed certificate is not as good as an authenticated
certificate, though, because there's no one to certify that you really
are who you claim to be. The user has to take your word for it. If
you use a self-signed certificate to sign your p3d file, the user will
be presented with a warning, and will have to go through an additional
step to approve your certificate. We recommend you use a self-signed
certificate only for internal development, but get a normal
authenticated certificate when you're ready to make your app available
to the public.
You can use the openssl command to generate a self-signed certificate.
You probably already have openssl installed if you're running on Linux
or Mac; if you're on Windows, you can find it on the internet easily.
A sample command sequence to generate a self-signed certificate
openssl genrsa 1024 > mycert.pem
openssl req -new -x509 -nodes -sha1 -days 365 -key mycert.pem >> mycert.pem