Putting it in a vise is way easier. I've got a couple pairs of soft Jaws but failing that, a thick leather belt is great.It can be somewhat tricky, especially the heavier springs of J & P. This is the way I do it, if the method above is problematic. Good point to use a correct fitting hollow ground bit, or screwdriver regardless. Getting the old one out is easy, getting the new one in can take some patients and energy without causing damage to the receiver side edge
I do it the same way as the Citori's. First before disassembly look at the position of the parts, and reference especially where the top lever retainer is positioned for the screw alignment when top lever is fully left. Now, get everything in position, and the screw pushed all the way into the top lever retainer. Hook the top lever retainer pin on the spring and put pressure on the screw while moving it toward the right for direct alignment into the hole. While holding pressure on the screw turn it backwards until you feel the threads starting point. You could also screw that screw into the receiver without the parts and reference the position of the slot at starting point. Then back it up about an eighth turn or so when assembling for a starting point. Make absolute sure that screw is aligned, or with that pressure you will cause a cross threading. Turn the screw while holding pressure in for at least a complete turn before regripping. Turn it in until tight, and it will compress the spring into position.
I do this while the receiver is laying on the bench, and on it's side. Putting it in a vice may be easier, and allow the other hand to hold the position of the top lever retainer. I use my left hand index finger, while holding the receiver in place.