Unfortunately there is currently no free signing process. The way to go is to self-sign your application.
This option is however only available for applications using no capabilities or Basic capabilities only ( LocalServices, NetworkServices, ReadUserData, WriteUserData, UserEnvironment and/or Location (since S60 3rd Edition FP2)).
If your application requires more capabilities, or if you plan to use Location capability on S60 3rd Edition or S60 3rd Edition FP1 devices, you will have to certify your application before release. The process is however restricted to companies and it is subject to a signing fee. See details at SymbianSigned.com