Just start a normal installation as you would allway do. But just after you've selected from which series to install you need to take notice. Here you would normaly choose an installation method: automatic, expert and so on. This time choose the "tagfile" option. You will be asked for the directory in which your tagfiles are. So now would be a good time to mount your floppy. You can do this by switching to a second console. First make a mount point (mkdir /pkn is what I use) and mount your floppy there (mount /dev/fd0 /pkn). In my case the tagfiles will be in /pkn/tagfiles, this will be the directory name I enter in the Slackware installer.
You will notice the installer starting to install the same packages as were on the model system. Now isn't that neat? Just like everything else with Slackware, it is not hard to do... you just have to do it yourself ;)