What to log, and why
Gravity before and after the boil tells you your efficiency and boil-off; fermentation temperatures explain most off-flavors; and packaging notes (priming sugar or PSI) let you repeat a carbonation you liked.
Keep every batch in one place: recipe, water, mash, gravities, yeast and pitch, fermentation readings, packaging and carbonation. Several batches save in this browser; print a filled sheet or a blank one for the garage wall.

ABV from your OG and FG: (OG − FG) × 131.25Worked example: Log water and salts, mash temperature and time, pre/post-boil gravity, yeast and pitch, fermentation temperatures, FG, packaging and carbonation.
Sources: AHA: How to brew
For guidance only: verify with your own readings, your equipment’s manual and the product labels.
Gravity before and after the boil tells you your efficiency and boil-off; fermentation temperatures explain most off-flavors; and packaging notes (priming sugar or PSI) let you repeat a carbonation you liked.