In a perfect world, all door casings would be in perfect alignment, and fitting a new door would be a simple job. However, in the real world of door fitting you will quickly come to realise that the majority of door casings are far from perfect, which can at times make fitting a door a challenge. In addition to this, there is quite a bit more to fitting a door than just getting it to fit the opening.
Doors are produced in standard stock sizes, and checks must be made of the door being replaced against the door chart provided to ascertain the correct size. It will more than likely be found that the door that is to be replaced is slightly smaller than the standard, which is quite normal; it simply means that the door was originally reduced to fit that particular opening. Because not all door casings are as one would like, a slightly larger door is in fact the ideal situation, as it allows the fitter to shape each door to fit. I should stress though, that the amount that can be safely taken off a door is minimal, so choosing the correct size to start with is very important.
Before the new door can be fitted, certain things must first be determined, such as; which 
edge of the door is going to be hinged, where the latch will be, and which end is the top? On a solid wood panel door, this is pretty straight forward, because all the components of the door is visible, and the bottom rail is always deeper than the top and the latch and handles can go on either side. This however is not the case with a 'Hollow Core' door because they are made in a different way to a solid panel door. As the name suggests, Hollow Core doors are not of a solid construction, so the manufacturer has to provide information on this type of door indicating the location of what is know as the 'Lock Block'. These marks differ between manufacturers, but in the main, the top rail of the door is stamped with: 'Top' or 'T', and to one side; 'LOCK', 'LB' or 'L'. This is telling you that the 'Lock Block' is either to the left or right hand of the door and that edge is where the latch and handles must be fitted. It follows that the hinges go on the opposite edge.
Getting Marked
With this in mind, and the door offered up to the casing with the hinge stile of the door to the hinge jamb of the casing, mark the letter 'H' indicating hinge side, 'LB' on the opposite side for lock and 'T' for top.
These marks are important, as it’s quite usual to shape and reduce the door in gradual steps, which means that it can be repeatedly offered up to the opening to check the progress. With the door marked in this way, every time it is offer up, you can be assured that it will always be the right way round.
The markings are also important when it comes to fitting the hinges, as the marks will serve as a constant reminder as to where the top of the door is, and from which face one should be working from.
I will later be discussing the fitting of the hinges, and will be referring to the 'face of the door', which is the face that has these markings on it.
If the door is to be stained or varnishing, pencil marks on your door could present problems. In this case it would be a good idea to put to strips of masking tape onto the door first, and place the marks onto the tape rather than the door. 
