Just pre-fill the new filter and put a pan to catch the very small drip that will occur when you change the filter. You won't need to add any to the transmission but if you haven't done the proper level check procedure in some time you should do so anyway because it may have been too high or too low when you bought the vehicle. The Allison 1000 requires the fluid be warmed to the proper temperature for an accurate check AND too much fluid will make the transmission run hotter.
There is a circular magnet that sits in the filter housing and this magnet sometimes comes off with the filter. It should always be wiped clean and put back in place and doing this makes sure that you don't accidentally throw the filter magnet away with the old filter. If this is a GM pickup, the Allison filters are MUCH cheaper at a truck dealership than at your GM dealer. Filters are cheap and I would replace once a year or at every other oil change, whichever comes first.
