You don’t need to scribe door .
If your old door fitted ok use it as a template to mark the new door
I tend to get it fitting in the width first, leaving it tight in the height, then scribe the top afterwards. If the variation at the top) is only a couple of millimetres this can be measured, marked out the door then planed in. When you've got the top and sides done you can tackle the bottom.
I tend to just hold the door in place with packers and wedges (or these days packers and Winbags) then eyeball the gap when I'm checking it - having a visually even gap round the three sides (left, top, right) can make even a barrel shaped door lining look better.
Remember that on fire doors the gap round the three sides must be 2 to 4mm (partly because fire rated hinges are set for a 3mm gap) whilst at the bottom the gap should be no more than 4mm. It may not always be possible to achieve this, especially if the floor is bad (e.g. goes uphill away from the door) so use your judgement. On real fire doors, or domestic doors requiring sound or draught proofing it may be necessary to fit a drop seal
I'd start by putting a 6ft level inside the rebates to gauge whether or not the jambs are straight, pin-cushioned or barrelled ,(the terms should be self explanatory). That gives you an indication of what too expect. I generally trim the width to fit tightly which at least allows me to "get the wood in the hole" so I can see if the head is parallel to the top of the door, or if that needs to be adjusted. How much you take off and in what order is really a judgement call. The approach taken can never be a "one size fits all" one
Yes, this really is an in and out process, because old jambs are rarely straight or perfectly parallel (IMHO a lot of new ones aren't anything to shout about, for that matter) it can't be a simple measure/cut/swing process. And the older the building, the worse it generally gets because of issues like settlement. That said most of the time I can do the job in two or three passes in the 'hole before my fit is good-ish (and on the rare occasions it goes in first time spot-on - but they are flukes ). All it takes is practice and patience
I was joking Keitai is doing this as a job and using DIYnot to ask all the stuff he can't do.And how do you suggest the OP gets into the corners?
And there's me thinking thats how you aquired your user nameI was joking Keitai is doing this as a job and using DIYnot to ask all the stuff he can't do.