The inside of the drain pan should be cleaned thoroughly. Usually a lot of gasket material sticks to the edges of the pan which can be easily peeled off by hand. It is a sticky paper-like material which is designed to stick and seal, this is why a little of effort has to be applied in order to remove all of the gasket material. A special knife might also be used, but it is important not to scratch the surface of the pan.
After all of this is done, brake cleaner will do the rest of the job, removing the remaining parts of gasket material. Note that brake cleaner is the only liquid that could be used on the inside of the drain pan. Do not use water, soap or other cleaners! The bolts that hold the drain pan usually get messed up badly. They should be cleaned with the brake cleaner also.
While the drain pan is drying out, you can move to the next step and deal with the filter: the old one should be removed and replaced by the new filter. There is no need in cleaning the old filter and trying to reuse it. If the transmission fluid is changed rarely as it should the filter has served for quite a long time and it can go to the dump or recycling with every transmission fluid change.
First of all, make sure that the O-ring is seated properly: it should be placed on the filter suction side and sit there tightly. Get under the vehicle and take the old filter out (it is located above the drain pan which has been removed before). Beware that there might be some leftovers of the transmission fluid left around the filter. After all the bolts are removed and the filter comes out, check the suction port to see if the old O-ring is out too.
Put the new filter in the place of the old one and screw the bolts back in tightly. It is recommended to start with the ones close to the suction port. Now the drain pan and the new gasket can be installed too.
It is possible to put the new gasket on the pan unattached and try to screw them back to the transmission, but it might cause troubles because the holes might not be lined up properly. To make the job easier, use special glue, gasket maker that sticks the gasket to the drain pan. As the glue is applied and the gasket is attached to it, put the bolts in all the holes to be sure everything is lined-up well. Usually gasket maker required 15 minutes to seal well, so set the gasket aside and wait after all the holes are checked.
Get under the vehicle with the drain pan and all the bolts, start them lightly and then tighten them in a special way: do not go in the circle, but screw the center bolts first on the top and bottom of the pan, then move in diagonal ways to be sure that the drain pan is set properly and will stick to the transmission as tightly as possible.
Although filter and drain pan installation require proper tightening of the bolts, be sure you do not over tighten them: first of all the bolts are usually made of aluminum and it is easy to break them, secondly, the threads of the transmission might become damaged.
Finally, the automatic transmission fluid can be refilled. The car owner manual gives advice on the best type of it for the vehicle. Remove the dipstick and use a funnel because the transmission tube is usually located far from reach. Three or four quarts of the fluid should be enough for the beginning. Start the engine and run the auto for a minute with the transmission in park. If the wheels of the vehicle are off the ground, it is good to run it through all the gears: Drive, Reverse and others. Check the level of the fluid with the dipstick to see if you have to add some more. As a rule automatic transmission of passenger vehicles requires 4-12 quarts of transmission fluid.