If the package already exist in the Debian repositories, you can get the .dsc file (for example in an ftp like this: http://ftp.debian.org/debian/pool/non-free/s/spim/ ) and execute the following commands:
dget -x DSC_FILE_URL
It will download the package and will unpack it in the current folder. You have to enter in the created folder and edit the debian/* files to personalize settings, mantainer data, add deps ecc...
When you're done, you can generate the package with the usual command:
dpkg-buildpackage -rfakeroot
