Capture at four moments
First visit or quote, the day you start, any stage that gets covered or changed, and the day you finish. Those four points cover most of what anyone will ever ask you about.
Write less, say more
Typing on site is slow. A short voice recording while you are standing in front of the work captures more detail in less time.
One place, not five
Photos in the camera roll, agreements in text messages and notes in a pad means three places to search. Keeping it in one place per job is the single biggest improvement most trades can make.