The MakeKeys tool can be used to create a private key-public key (in form of a self-signed certificate) pair. The private key can then be used by the How to guide for creating/signing sis files tool to digitally sign a Symbian installation file (.sis). See Deploy SIS file on hardware for an overview of the tool used in this context.
The self-signed certificate can be used by the MakeKeys tool to create a certificate request file, which can then be submitted to a Certification Authority (CA) for signing.
More information, including reference and a tutorial, is available in the Developer Library.