if you do not have a conical bowl then your air hole doesnt come in at the bottom of the bowl, meaning that tobacco will not have a correct orientation to the draw. You can grind a spade bit into a conical bit pretty easy.
I prefer to drill the airway first, and then carefully drill the tobacco champer down to it, checking every 1/8 of an inch when I get close as to not over drill it. But this is preference. It is my advice however to order some kits for your first few pipes. Then the mechanics are all right, and you can focus on making it pretty.