you need to change xyz.sis to the name of your unsigned sis file, xyz_signed.sis to the name you want the signed output file to take, xyz.cer to the name of your .cer file, similarly for xyz.key, and 12345 to the password you used when you created the .key file.
As it's a batch file which will execute by default when you open it, you need to right click on the file and select 'Edit' in the pop up menu which appears, to open it in Notepad. Make the necessary changes, save the file, make sure all other dependent files are in the same folder, then run it and the signed file should be created.