I'm assuming that if you can access your CUPS at localhost:631 then you have your CUPS server running. Since you're new to linux and slack: there is a file /etc/rc.d/rc.cups that has to be executable so it can be run at boot. It can also be run manually from a console: usage is /etc/rc.d/rc.cups sto, or start, or restart. If you change any cups setting manually run "/etc/rc.d.rc.cups restart" as root.
Now..........assuming your printer is a usb printer and turned on: As root in a console run the command "lsusb". Is your printer listed?
In a browser go to:
http://localhost:631 Opt to go into Administration and when prompted log in as root select "Add Printer"
Enter a name for you printer then hit "Continue" There will next be a pull down list and if your usb printer is on you should see it listed.
Select it and pess "Continue" if it's listed, if not listed you need to find out why.
Next Select your manufacturer then from the list you printer or the nearest best printer. For example I use PSC2350 for my HP PSC2355.
Continue on to the next page where you should be advised that your printer (name) has been added successfully.
Click on the hyperlinked name of your printer then configure for papersize, resolution, etc. then click "Continue".
You should be then able to click to print a test page.
Look to see that there is a button that says "Stop Printer". That means that the printer is started. If it says "Start Printer" that means it is stopped.