Credits for this information go to this site. Although it is dedicated to the Nokia N91 phone, following the instructions has allowed me to succesfully sign and install my SiS files on a Nokia N80 phone. I suppose it works for other models too.
Download and Install the following packages:
Developer Certificate Request Tool for Symbian
Sign SiS tool
Phase 1: Generating a Certificate Request
Run the Developer Certicate Request Tool
In the first screen enter the destination and name of the certificate to be created (e.g c:\mycertificates\mycert.cer).
In the second screen choose No in the ACS Pub ID Avaialble field.
Then enter the destination and name of the private key to be created (e.g c:\mycertificates\mycert.key). Also provide a password for the private key and click Next
In the following screen enter your personal details. Hint: no need to enter your true details.
The next screen asks for your phone’s IMEI number. This can be retrieved
by pressing *#06# in your Nokia phone’s main screen. Be carefull to input this
accurately otherwise all this effort will be wasted and you will find out
after the whole procedure is over. Select the Application capabilities you
are interested in and add them to the right hand side of the given table.
In the next screen review your certificate and click finish.
You have now created a request file (mycert.csr). This will be used later to acquire a certificate (mycert.cer) with which you will sign your files.
Phase 2: Acquiring a Developer Certificate
Go to the SymbianSigned and create an account.
Once you create your account login and click on the “My Symbian Signed” tab.
In the left menu click Open Signed” and click the “Request” link.
Enter the appropriate code and then browse to your certficate folder and choose the mycert.csr file. Then click “Send”.
After a while the certificate will be generated.Download it and save it.
Phase 3: Signing an unsigned SiS file.
Run the SignSiSUI application.
In the first field select the SiS file to be signed.
In the second field select the certificate that you downloaded from Symbian in the previous phase
Finally select the private key that you have generated (mycert.key) and enter the password.
If everything goes fine, the SiS file will be signed successfully and you can now install it on your Nokia phone.