This is an interesting question to me because I haven't had to CAD As-Built/As-Build anything in my career until recently. I also never did any of this in college. All the things I have done in CAD have been for new items; never for parts that exist.
My thought was that I would buy some calipers and go to town measuring the parts. However, real parts do not have center lines or nicely laid out dimensions, obviously! My first few models were junk, pure junk with "accurate" measurements.(?) I would make sure to take many different measurements of the same feature to get the best results possible. To get these measurements I would do things that I would never do when creating new parts. Because of that, I would end up making a second model of the same part but this time using better CAD etiquette.
After finding out that there is an art to measuring, I went on to the net to find better ways. Sadly, I am only finding articles and videos of people showing how to use the calipers, not the best practices when a part is sitting in front of you. Now when I look at a real part, I have a "Reverse Design Intent" in my mind before I start doing anything.
Do you have any suggestions to help model real parts? Or maybe even websites to doing this? I know that I should have a bunch more tools than calipers. What would be good tools to get measurements?