Two ways. Type them one at a time: after each save the date jumps back a day, so you can go straight down your old notes. Or paste a list with one date and weight per line, in any common format.
Once a month, same time of day, tape snug but not tight. Waist at the belly button tells you if the weight is mostly muscle or fat.
Five minutes. Don't grade yourself, debug the system. If something kept slipping, change the plan or shrink it, not your opinion of yourself.
What the book says, how strong the research behind it is, and where this board uses it. Proven means many studies agree. Partial means some support or mixed results. Untested means it's a good idea nobody has really measured.