With hand tools this is where a good bench comes into it’s own. A traditional English bench will have about 6 to 8” at the front which is good solid timber that has been straingtened and flattened. For making stuff you don’t need a big flat area such as you see often promoted, the on,y bit that needs t9 be thick and solid is where you will actually plane. No one planes over the entire surface of their bench....
Most use winding sticks and the longest level you have besides the mark on eye ball to get the bench top flat and true. The reason the bench top is key is that as you plane the stuff it will bend and form to the shape of the bench top. If it’s not flat and true forget trying to plane anything of any length straight upon it.
First use either a proper scrub / modified no 4 or a No 5 or 5 1/2 with a good curve on the blade to knock off all the high spots and to start and get it flat and out of wind. For me, the easiest reference is the bench top, I just flip over the stuff onto the planed side and look for gaps between it and the bench. Stops all the messing around with winding sticks and sighting etc. You knock down both (planing across the grain generally) sides until you end with something that’s reasonable level, flat and true. It’s shouldnt rock on the bench and most of the stuff should be in contact with the bench. It’s actualky very quick to do with the right plane. Probably no more than 10 mins work (5 mins a side if your fit) Now use a no 6 or a 51/2 or a 5 at a pinch with a blade with a normal curve to it. Now plane one side flat and true removing the marks made by the first stage. You can use winding sticks but again with this section of wood the bench top is far easier to use as a reference.
(A good planing stop is essential, for me it’s none of the modern stuff, it’s a batton simply nailed across the end of the bench. A bench top is a disposable tool)
Now flip the stuff over and repeat for the other side. Now flip it back again and re-skim the first side. Remember the wood moves to the bench top so any hollows or bumps on the face that is against the bench will translate slightly to the planed surface. The second skim removes the last of the imperfection.
For many facing both sides at this stage will seem to be unnecessary, however as it’s a door I don’t want it twisted or slightly bent, I will one side as my reference for all subsequent joinery and it needs to be as flat and true as I can make it.
Use a gauge to mark the thickness all around the stuff from the side that’s had the second skim. Plane down to near the mark with the scrub and finish with a no 6. As you hit the scribed line you should see it in the shaving you are producing. If you have a proper scribe mark you can easily see it in the shaving. Hard to explain, but when you see it you will know!
Repeat for the other two sides.
It’s a great work out, I do it once in a while just because I can!
I totally agree with the remarks about the absolute rubbish door frames that get fitted. There is absolutely no reason for it, it’s just poor workmanship and sloppy practice. A well made door frame inserted properly after the house has dried out that is properly painted all around will hardly move over time. These days I replace both together as it’s faster and simpler and produces a far better result.
To check that the frame is ‘good’ and is worth saving tap in a nail at each corner on the outside of the frame. Now run a string diagonally across each pair of nails. The string should just touch where it crosses in the centre. It is doesn’t, the person who installed it wasnt very good and the door frame has been installed in twist. You will never get a satisfactory fit with any door. If it passes this test next check that the veriticles are plumb on both sides. If they are brilliant. If not again this is poor fitting. Next, assuming it passes measure the gap where the door fits at the top, middle and bottom. The measurement should be the same. Lastly check the head of the frame to see if its level, if it is, keep the frame and make the door to fit. If any of these checks fail it’s easier to make and fit a new frame than try to correct the bodged up work.