dh_link accepts a list of pairs of source and destination files. The source files are the already existing files that will be symlinked from. The destination files are the symlinks that will be created. There must be an equal number of source and destination files specified.
The list can be specified in two ways. A file named debian/package.links can list pairs of files. If you use this file, you should put each pair of files on its own line, and separate the files within the pair with whitespace. Also, pairs of files can be specified as parameters - these pairs will only be created in the package build directory of the first package dh_link is told to act on. By default, this is the first binary package in debian/control, but if you use -p, -i, or -a flags, it will be the first package specified by those flags.
Be sure you do specify the full filename to both the source and destination files (unlike you would do if you were using something like ln(1)).
dh_link will generate symlinks that comply with debian policy - absolute when policy says they should be absolute, and relative links with as short a path as possible. It will also create any subdirectories it needs to to put the symlinks in.
dh_link usr/share/man/man1/foo.1 usr/share/man/man1/bar.1
Make bar.1 be a symlink to foo.1
dh_link var/lib/foo usr/lib/foo \ usr/share/man/man1/foo.1 usr/share/man/man1/bar.1