When shooting at a monster and its rider, roll to hit the model as normal, adding +1 if the monster is a large target. Once you have established bow many hits have been scored you must apportion them between the rider and me monster. For each hit scored roll a D6; on a roll of 1-4 the monster has been hit. Roll to wound the monster as normal. On a roll of 5 or 6 the rider has been hit. Roll to wound the rider as normal.
Work out wounds separately on the rider and his mount. Take any saving throws clue to the target as normal. Most monsters do not have an armour saving throw as they have no armour, but some have scaly hide which confers a equivalent save. Riders are permitted saves for their armour, but remember that they do not receive the additional +1 save as cavalry troops do when riding smaller creatures.
If a monster has two or more riders (a very unusual combination) then randomise hits to see which rider is hit and work out any resultant wounds on that individual. If models are glued in place (as is likely) it will be necessary to make a note of any casualties suffered.