I'm a DIY-er also. But I'm fortunate enough to be able to take the more valuable horns to the pros when necessary. I only try major things on "project" horns of little value and I only do simple, routine things on the more valuable horns. This guy was mentioned above, but he has a very good series of videos on YouTube.....
There are also many other good tutorials on YouTube.
My most recent project has been lapping the valves on my Bach Mercedes. Before I started, that horn had the stickiest, balkiest valves of any horn I ever played. If that was really a Strad valve block before it went into the Mercedes, it must have been a "second" or reject block. I still don't have the valves the way I want, but they are much better than when I started. To avoid over-lapping, I only do a little hand-lapping and then clean it out thoroughly, play for a few days, and repeat. Nothing appears to be bent or warped. A pro could probably have gotten it right in one operation instead of several trial-end-error sessions, like me. But what the heck. I enjoy it and the horn is only a backup and practice instrument.