First of all, the RS, RW etc. are called signals (if we're talking protocol) or pins (if we're talking pin-out), not channels.
Now, like coorz said, you have to find the schematic you used to wire the LCD up. There, you will be able to see where these pins are connected on the LPT. Fill those in appropriately, and you'll be ready to rock.
Live long and prosper...