You need to fit the door lining perfectly so when you come to hang the door it's straight forward, quick and simple. A correctly installed frame needs to be; 'In wind' means that both jambs or legs are parallel with each other. If they're not in wind they will be twisted slightly, meaning when you close the door the top will close before the bottom, or vice versa. You've probably come across a door that's had this problem at some point it's fairly common! Sometimes it's noticeable because when the door closes it touches the stop but doesn't click shut, and you have to push it harder for the latch to locate (this could be ill fitting door stops too). What is happening here is the very top of the door or the bottom is hitting the door stop before the rest of the door, meaning the latch hasn't reached the point at which it will click shut yet. Pushing harder flexes the door enough to latch it shut. If you have bought a pre-made frame, the door lining head should be housed out either side for the legs to slot into.
Sometimes the housing is the same thickness as the leg, other times only half the thickness is removed and the top of the lining leg has a half tenon that slots into the head. In this case, make sure you put the legs in the correct way around to allow for your doors width. Generally doors start at 2ft and increase by3 inches (2'3",2'6", 2'9") Personally if given a choice i'd always choose the lining where the housing is the same thickness as the leg (above left) because if the leg happens to get damaged or there's a nasty knot/shake in one side you can just spin it around and use the good side - not possible if it will only work one way around. The head of a door lining set is also often housed out on both sides to accomodate two different sizes of door. If you are making a door lining up yourself from stock timber add around 5mm (1/4") to the width of the door to ensure a gap either side of the door when it's hung. Determine and drill through the housed section you want.
I put 3 clearance holes in each slot. Then using plenty of PVA wood glue assemble the lining and screw the legs in with 80mm screws. Finally, cut the excess timber off the head (you can leave a bit of this on if the opening is much wider as it may help keep the frame in place while you work). It;s easier to install if you've already braced it in a perfectly square position. If the frame you are installing is a hatch like a loft trap you can check it is square by measuring from corner to corner (measurements should be identical). If fitting a normal door frame, it's far easier to install if you first brace the legs at the bottom so they are held the correct distance apart and also brace the top corner of the frame diagonally. This will keep it held square while you're working on it in the opening. It also makes it easier to move into position as it's not trying to pull apart etc. To brace the bottom at the same width as the top, grab a piece of batten and cut it to the same overall width as the head (outside to outside measurement of the legs).
Mark with a tape measure 150mm (6") up from the bottom of each leg and nail the batten to that line, flush with the outside. This will keep the bottom of the legs the same distance apart as the top is. Some carpenters repeat this on both sides to keep the legs really straight. Next, grab another batten and fix it diagonally from the head of the door lining to a leg like in the pic above, to hold the top at a perfect 90° angle. To do this either use the 3 4 5 method or a large framing square to hold it at 90° (more accurate/quicker) while you brace it. (Nail toward the outer edge without splitting so the architrave will cover the nail holes up later). When the frame is braced up it should look like the one below, Place the door lining in the opening and check that the head is level. I always mark the hinge side straight away so if I remove the frame to work on it, I know which way to put it back in. If the head isn't perfectly level put a wedge under the side that needs to go up until it is perfect.
Measure the gap under the wedged side, and cut that much off the bottom of the other leg. This should make the lining sit level when you put it back into the opening. The hinge side is the most important one to get straight, solid and level at this point. When fitting to studwork, I drill and countersink 8-10 holes in each leg, in pairs - 100mm down from the top, the same up from the bottom and then equally spaced in between. By screwing the frame in place you can adjust it by winding the screws in/out later and wedging behind it.If the wall you are fitting to is masonry you'll want to space the screws so they fall in the middle of the brick or block instead because too close to the edge of a brick and it'll just split or shatter. It's not always easy to get a decent fixing in the cement joint either. If you won't be able to see the bricks when the lining is in because the wall is a previously plastered one mark the center of the bricks on the face of the wall or lining before sliding the frame in.
Use 80-100mm long screws and red/brown rawl plugs. Next, I fit the lining in position and screw the hinge side up first, making sure it is perfectly level and equally overhangs the wall behind. Put a 1800mm spirit level on the face to check it is straight, and use either timber packers (I rip a load of different size packers down) or folding wedges like in the picture on the right to wedge out any hollow areas. Using a spirit level held across the face of the lining like in the drawing below (it's a birds eye view), make sure the frame is parallel with the wall, and there is an equal overhang before screwing the top of the other side. This is important because the plasterer will want to use a straight edge exactly how you are using a spirit level now to make sure the wall is perfectly flat. If it's a studwork wall it's still important because the door lining needs to maintain the 'flatness' (not sure that's actually a word..) of the wall when the plasterboard is screwed on. Then, sight the frame through to make sure it is 'in wind'.
This is done by sighting one edge of the door frame through to the opposite edge like below. As you look through, let your eye wander from top to bottom and check the legs are parallel. The picture below shows a good and bad lining that doesn't sight through/is out of wind. Again, this is important because if you get it wrong the wall won't be flat and when the door is hung it won't close properly. The top will either close before the bottom, or vice versa. Occassionally you'll come up against a situation where several door linings need to be installed in a hallway or corridor and they'll all need to be the same height and perfectly in line. It's more common in commercial projects, and buildings like schools/hospitals etc. To do this I fit one door frame correctly at each end, and then pull string lines tight from one to the other. This makes installing the frames in between a lot quicker and easier as you simply put them up to the lines and screw in place, before packing out to make sure the sides are straight.