Fitting
Begin by removing the existing door and detachable threshold.
When the fitting procedure is complete, a 3mm gap will exist around the top, and both sides of the door. Initially though, we will only be concentrating on fitting the door into the opening, so that it's a snug fit all round.
The likelihood is that you will be fitting the new door to the existing frame, so that the recessing for hinges may not be necessary. However, if you find that the existing cut-outs are not suitable, or do not match the new hinges in some way, you may need to 'make good' the areas first by splicing-in new sections of timber.
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Personally, I first evaluate the shape of the door frame by offering the door up to it, and 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 may exist in the door frame giving me a good indication of what will be required and the easiest way of making the door fit.
If a door frame head is out of level, or the frame leg 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 adjustments 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 for it to fit into the opening 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 frame head, and find that both edges are not parallel, scribing
is of the door top rail will be required. Keep the door in this position by placing wooden wedges under it, and measure the gap at the far edge of the door and frame head (at the point were the gap is greatest). If this gap is 6mm, 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 frame 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 frame head, the pencil will scribe the required line across the door so that the pencil line and the door frame head will be parallel.
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 3mm 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 3mm packing pieces on each side of the door.
Mind the Gap!
The next step is to consider a suitable gap at the bottom, however, as this is an external door with a detachable aluminum weather bar, the required space under the door can only be determined after the hinges have been fitted and the door is swinging. Therefore, it is only required at this stage that there is a gap under with would allow it to be opened.
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