Just mount the disc to a directory. Then cd into that directory and use installpkg
as root to install any package you want. Example:
Then cd into the slackware directory of disc 2. There are individual directories in there for each of the software series. You can cd into each one and run:
to install any of the packages. Or if you want them all, just cd into each directory and run:
and all packages will be installed. Repeat this for disc 3 if you want KDE.