The simplest way is to go to Control Panel / Printers, Choose you printer then right click on it and select sharing and choose a share name.
If the printers upstairs are running a different operating system then you can install different drivers at this point.
Now go upstairs and on each PC go to control panel / printers / add new printer. Click the network printer option and find the printer on the downstairs PC and follow the instructions.
Note that the computers upstairs will only be able to print if the downstairs PC is switched on.
If you want to be able to print from any PC regardless of which PC is switched on follow this thread click here