Rear Differential Pinion Seal replacement

[catch-22]

I replaced the rear differential gasket on my 1996 GMC sierra a few weeks ago only to find another puddle of oil. I will be replacing the pinion seal next - something I've never done before. Is there any hints or tricks I should know that might make the job easier? I've read that you have to torque the pinion nut back on to a specific pressure - is that true and what is the torque value? Should I put thread lok on the pinion nut? Any help and/or comments appreciated