Ok, easy then, Pinks... and thanks for your kind comments :o)
What the others have said really. Punch a hole in one side of the plasterboard somewhere in the middle of where the door is going. This will enable you to find where the upright studs are. This is so you can chose a position that involves cutting as little studwork as possible.
Standard stud spacing is either 400mm centres or sometimes, 600mm centre to centre. Taking out one upright should allow you to fit a standard door lining with a 2'3" door. For a 2'6" door, you'll have to take out another upright (stud).
This is all theoretical, of course. It may turn out to be different when you get there.
Don't take the studs out completely, but cut them so that a crosspiece (head) can be fixed in to become the head of the doorframe.
Where possible, cut the plasterboard with an old saw to give a nice straight line. With care, it should be possible to fit everything together without having to patch up the plaster. Without the benefit of experience, you may find some patching up is needed though.
With a good saw this time, cut through the sole plate (bit of studwork that is fixed to the floor.)
The trick with this job is to disturb as little as possible. The TV programmes always show people laying into a wall with a sledge hammer. That's fine. You'll break through Ok, but will have twice the job with all the patching and finishing.