$ cd my-repository
$ dpkg-scanpackages binary /dev/null | gzip -9c > binary/Packages.gz
$ dpkg-scansources source /dev/null | gzip -9c > source/Sources.gz

The Release files are simple and short text files of the following form:

Archive: archive
Component: component
Origin: YourCompany
Label: YourCompany Debian repository
Architecture: architecture