Mike, it kind of depends on the door. Most of the time they are built to have some inches taken off the bottom. If it is a hollow core door you need to determine if the bottom in solid beyond the amount you need to take off. You don't want to cut it and have this hollow space in the middle.
Manufacture should have specs. If not you could try tapping it with a tool and listen to the changes in noise.
Plenty of advise online on how to make the cut. Here is how I do it.
I put a piece of masking tape or painters tape over the line where I am going to cut. I also you a razor blade to score on the line where the saw will cut (that scoring should be on the edge of the curf towards the door, not the discarded trim piece) I make this scoring on the two edges and face where your saw will be sliding across. You don't really need the scoring on the bottom as the circular saw blade is cutting towards the wood.
You do all this, because a circular saw cuts up towards the saw and it may very well chip out the wood on the outside of the door leaving a chipped up edge.
Use a piece of wood clamped to the door to guide your circular saw for a straight cut.
Here is one https://www.youtube.com/watch?v=IMeACLrOtVs
I usually look at several online before jumping into something new. He scored it on all sides which is fine. A straight edge to guide your saw will make a difference.
This guy has an excellent guide. But I would still score the door on top and edges.https://www.youtube.com/watch?v=R3pQplY3qIA
You can just use a straight edge clamped to the door. But you must know the distance between the side cut of the saw blade and the edge of the saw that will slide against the guide. This distance lets you know where to clamp your straight edge. For example I clamp my straight edge 5 3/16 inches away from where my cut will be made. Best way to measure this distance is to use a scrap piece of wood clamp a board 90 degrees to it and start a cut. Just measure from clamped board to edge of cut.