The filling is really up to you- it is simply a meat pie with a mashed potato top. You can make the potatoes and filling ahead of time, but leave it out to come to room temp before cooking if you do. Otherwise you will wind up burning the top and have a cold middle.
Once you get the amount of time and amount of tender quick perfected for whatever type/size of meat you are using, then the seasonings can be increased. I recommend that you make a half recipe of this if it is your first try.