When fitted there will be a 2 mm gap around the top, and both sides of the door, with a 6mm gap to the underside, but this gap to the underside could be different depending on the type of floor covering. Initially though, we will only be concentrating on fitting the door into the opening, so that it's a snug fit all round.
Personally, I first evaluate the shape of the casing by placing the door up to it, and by trying it in different positions. This is useful; because I know that the new door will have square corners and parallel sides, and will show up any imperfections in the casing, giving me a good indication on the easiest way of reducing the door to fit.
If a door casing head is out of level, or the casing let is not vertical to it, this very often means that the door head will need to  be reshaped to fit by scribing. However, if these differences are minimal, it is sometimes possible (and easier) to make this adjustment along the edges of the door rather than at the top by tilting the door slightly.
You may also find that the door height is greater than the opening height, in which case the door will need to be reduced, otherwise the shape of the top of the door and casing head would not be visible for evaluation. But even with the door being too tall, you can still get the general idea of whether the two differences are great or small.
Ultimately though, the height of the door will need to be reduced, but at this point, great care should be taken that not too much is removed from the bottom, because if you later find that the head also needs scribing (which will also reduce the door height), the door could end up being too short when the scribing is done.
There are a number of ways going about reducing the door to fit, where the more experienced would place the door up to the opening and remove the appropriate amount until it fits, but if this is your first attempt, it would be safer to measure the width of the opening across the top, middle and bottom and transfer the sizes on to the door. The door can then be gradually reduced to the correct width by planing equal amounts from each side so by maintaining its symmetry.
If you find that after fitting the door into the opening, and raising it up so that it touches the casing head, you find that scribing  is required. Keep the door in this position by placing wooden wedges under it, and measure the gap between the door and casing head (at the point were the gap is greatest). If this gap is 6mm for example, remove the door and place it onto the trestles, and measure down 6mm from the top of the door on opposite side, and draw a diminishing line from this mark top of the door on the opposite side. An alternative to this would be to leave the door in situ, and draw a scribe line across the width of the door, while at the same time maintaining the same distance from the casing head so that the pencil line and casing head run parallel. To aid this procedure, a simple hardboard or thin plywood guide as shown here on the left could be used, so that when the top of the guide runs along the underside of the casing head, the pencil will scribe the required line across the door.
Leading Edge
With the door a snug fit in the opening, and the top of the door and casing head parallel, you can now confidently remove the  required amount from each side of the door so as to achieve the correct 2mm gap to each side, but don't forget to remove the leading edge of the door on the latch side, otherwise it may be found that the door catches when closing even though it fits correctly in the closed position. This problem is due to the increase radius of the door at its leading edge. When planing the closing stile of the door, this should be done so that it forms a slight angle to the full length of the door towards the outside face of the door by about 2mm. When complete it should be possible to fit 2mm packing pieces on each side of the door.
Mind the Gap!
The next step is to provide a suitable gap at the bottom, this is usually about 6mm but can vary according to the different thickness of floor coverings. Assuming that the gap is going to be 6mm, raise the door off the floor so that a gap of 2mm exists between the top of the door and casing head and place wedges under the door to keep it in this position. Ensure that the door is parallel to the casing and measure 6mm from the floor onto both sides of the door. Remove the door from the opening and place it onto the trestles and using a suitable straightedge, draw a definite line over the scribed line ready for the cut.
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