Alright, we had a damaged door over at our church . This door was custom installed ,by me , because it isn't a 6'8" H but 63 3/4" H.
Here is the cut off door beside the original door ,yes they are the same height just not from the angle I took the picture.
At a home improvement store you can just buy a door , without hinges or door jam or prehung . So the door needs to be cut off . Measure and mark the door , then take a sharp knife and cut thru the outer layer of lauan . Turn the door over and door the same thing.
If you don't and you use a hand saw or power saw with a fine cut you will splinter out one side of the door. .
As you're cutting the door , I used a clamp to hold up the cut side as I was cutting across the door.
Once you get to the end of door, then hold that side so you don't splinter underside of the door.
Now you have the bottom of the door cut off , and you can see how your door is constructed inside.
As you can see it is hollow with a cardboard type material glued in place giving some support or strength to the door.
Now you have a choice , either cut a board to fit inside this door, or remove the board from the bottom of the piece you just cut off. I chose to cut the piece I just cut off , At the bottom of the door (or piece just cut off) apx 1 1/2 -2" up this piece of wood you need is up inside the door.
I cut the bottom part of the door off .
Now you need to remove the lauan finish glued on at the factory to the front and inside of the door
This piece of wood with the layer of lauan are glued but not all that hard to remove . I used a chisel and light tack hammer to remove the layer of lauan.
Once the piece is free of the lauan , take your chisel to or long utility knife to cut the glued pieces inside the door and bend the cardbord material enough to allow your piece of wood to fit inside the bottom part of the door you need.
Before starting I had both doors laying of their side and even (or square) at the tops. Removed the hinges made sure the outside of door was against the back of the other door. Or both good sides was (one with hole) and good side of new door pointed the way . I then marked the location for the hinges with square , and utility knife. Tool a chisel marked the outer side of the hinge ,then made cuts like a shutter in the location of the hinge. . The depth of the cut is the height of your hinge. The top of hinge should be smooth with outer edge of your door.
I didn't show , but the other edge of the door ,has a door keeper, the other part the lockset is on the door still at the church.
The door still needs to be sanded , then stained , hardware installed and then taken to the church.
Hope this gives a idea of what the inside of most doors or in most of our homes. Doesn't matter if door is smooth like this one, a panel door . Most are hollow core. Yes you can buy a solid wood door , but most of the contractors don't use them because of the cost.