Mike, I don't know whether you intend replacing the window-runner felt 'in situ' i.e. with the windows still fitted to the doors or with the frames completely removed. If the latter, read no further and press 'return' now.
I did mine 'in situ' and actually quite enjoyed doing it. The one tip I would pass on, which worked for me, was to chamfer the bottom leading edge of the new 'U' rubber and I added a blob of grease for good measure. The idea was to be able to ease the new rubber over a minor obstruction in the window channel - possibly a stud or rivet or screw head which, from memory, is a few inches below the top of the door. Once past the obstruction, the rubber slides easily to the bottom. Without carefully cutting a chamfer, with a Stanley knife or similar, the new rubber just came to a dead stop i.e. nowhere near the bottom of the channel.
Good luck!
Alan