Check the frame is square (ish) and firmly fixed, measure at at least 3 or maybe 4 points width and height- you should be able to buy a stock size door – or get a blank cut to size
When you have the door it makes sense to treat the bottom with a sealer to prevent it wicking up moisture (if it is an outer door. It would also need a drip bar fitted.)
1. Stand the door in front of the frame. If the door overlaps the frame or is tight between the door and the frame, then mark with a pencil where the adjustments are required. There should be a 2mm or 3mm gap between the door and the frame on both sides and the top. If you do need to cut the door down to size, keep in mind that you need to cut all four sides of the door equally whenever possible to retain an accurate shape or pattern if the door is panelled. AND seal these cut edges.
2. For small adjustments in size, use a plane to shave the wood. Always start at the outside of any door edge and plane in towards the middle of the door thickness. If you need to make a more substantial adjustment it is advisable to use a panel or circular saw. Please be aware that circular saws are quite capable of (jamming) kicking back. It should be obvious, but isn’t always, you should never place your hand anywhere near the side or the front of the circular saw blade.
3. If you intend to use the existing hinge recesses in the frame, the positions need to be marked on the door frame. Position and support the door at an angle to the upright and mark on it where the tops and bottoms of the hinge recesses are for each hinge. wedges are handy to lift the door to the correct height, accurate marking is essential 3mm out and it’s a cock up, also make sure you cut the hinges in in the right direction – it is so easy to go out rather than in and make a mess of it. AMHIKS. If you are unsure you can use a piece of timber trim to mark off and transfer the marks. If the old door in on site you could use that (providing it fitted)
4. If you are to use new hinges you will need to mark their positions on the frame and the door. Exterior doors should be hinged between 125-150mm (5-6 inches) from the top and 175-230mm (7-9 inches) from the bottom of the door. Exterior doors are heavier and require a third hinge in the middle of the door. If the door is made of hardwood then it is best to use brass hinges sized 100mm (4 inches), 125mm (5 inches) or 150mm (6 inches). If the door is a fire door it must have CE marked hinges – if not use the best you can find it will save hours of frustration down the line.
5. Care should be taken when chiseling out the recesses for the hinges, there are may ways to fit a hinge – My preferred method was to fix the hinge with a size smaller screw into the correct position and cut around with a Stanley knife (other knives are available)to a reasonable depth remove, then with a very sharp chisel chop a series of strips 3mm apart along the length of the hinge from top to bottom inside the knife marks. The using the same sharp chisel from the edge of the door clear out the wood, level and try fit frequently until the metal is flush and flat with the edge of the door. Always make pilot holes when screwing into wood as this will prevent the wood from splitting.To make the screws go in easier, rub the screw thread with a bar of soap.
Make sure that the hinges fit their recesses snugly.
6. Once the hinges are attached to the door, screw each hinge into position using one screw only. Check that the door opens and closes properly before screwing in the remaining screws. If it does not, then unscrew and make minor adjustments to the position of the hinge, within the frame. Repeat this procedure until the door position is correct.If the door has a square edge and depending where in the country you live the leading edge needs a chamfer as square door- square frame = bang every time it closes- we call it shooting in the door but other use differing terms. just a mm or so outside chamfering to inside.
7. Finally, and only if you have done everything correctly, congratulate yourself and open a can of well deserved beer.
Fit locks and seal any raw edges – advise the door should be sealed if exposed.
Here is a link to another way to fit hinges: http://www.finewoodworking.com/item/33481/how-to-install-butt-hinges/page/all