If they have a tar.gz use that. unpack into an empty dir and have a look. Then use makepkg to build a Slack package.
Simply "having a look" won't be much useful, because if you download a program in tar.gz format, you'll mostly find the sourcecode inside! these instructions work in many ways, if you watch the output.
tar -xzf program.tar.gz
make install DESTDIR=yourbuildpath
(everyting without root permissions, so you detect errors, if this scripts wants to install some files into the main filetree, instead of the place where you want to build your package)
this should compile and put it into yourbuildpath. then you take a look and build your .tgz with makepkg.