I've just completed the very same repair on my daughter's T42P using the step-by-step FRU replacement instructions in IBM's PDF hardware maintenance manual for this machine. Fortunately every Thinkpad T-series teardown/rebuild I've ever done has been absolutely straightforward as the instructions are everything they should be.
On the negative side it's a very lengthy procedure as you have to remove every single device including the system board, before you can remove the old power socket.
There are lots of small screws so make sure your work area is clean and uncluttered and well organized before you begin. Don't worry about labelling the screws as they come off as the manual tells you exactly what screws to use, and exactly where they go, for each piece during the rebuild.
Make sure you have some good quality cross-head screwdrivers as many of these screws are tightened with a fair bit of torque and if the blade doesn't fit very well you'll end up destroying the head of the screw and making it difficult to remove.
I also recommend you get a bag of replacement screws to make sure you have replacements on hand for any that get mashed or lost. For T42 series this is FRU part 13R2916.
One other thing. If the laptop has been without power for some time, the small "backup" (CMOS) battery may have gone flat and you'll need to replace that too or else BIOS settings won't persist from one boot to the next.