A scoresheet is three columns and about sixty numbered lines, and in my first tournament I filled two of the columns in the wrong order for eleven moves before an opponent pointed at it. Filled in properly, it looks like this:
White Black
1. e4 e5
2. Bc4 Nc6
3. Qh5 Nf6
4. Qxf7#
1-0
The number on the left belongs to the pair, not to one move. Move four in that game is White’s Qxf7 and whatever Black would have answered, which is why a game “twenty moves long” has forty moves in it. You record your own moves and your opponent’s, so the two sheets should come out identical, and at the end you both sign both of them.
Write it after you play it, never before
The laws are blunt about this. Each move goes down as it is played, as clearly and legibly as you can manage, in algebraic notation (Art. 8.1.1), and writing your move before you play it is forbidden. The single exception is a draw claim under Art. 9.2 or 9.3: there you write the move you intend to make, do not play it, stop the clock and call the arbiter.
You are allowed to answer first and write afterwards (Art. 8.1.3), which is what most players do when the position is sharp. Play, press the clock, then fill in both boxes. What you cannot do is make a second move while the first one is still unrecorded.
Two other things belong on the sheet. A draw offer is marked (=) by both players (Art. 8.1.4), which is how an argument about who offered what gets settled an hour later. And your clock times, usually written beside the last move before each time control.
Legibility is not a courtesy
Your scoresheet is evidence. A threefold repetition claim, a fifty-move claim, a dispute about an illegal move: every one of them is settled by somebody reading it. If the arbiter cannot tell your b6 from your h6, the claim goes nowhere. The sheet is not really yours either. In most events it belongs to the organiser (Art. 8.3) and the top copy is collected at the end.
The habits that help are small ones. Capitals for pieces and lower case for squares, so Bb4 and b4 never blur. A closed 4 and a plain 1. And write the result at the bottom: 1-0, 0-1 or ½-½. Sign it with your eyes open, because a signed result stands even when it is wrong, unless the arbiter decides otherwise (Art. 8.7).
When you fall behind
It happens in every long game. You get absorbed, play six moves in a rush, and look down at a blank sheet.
Catch up on your opponent’s clock, from the position and your memory, and do it before you play anything else. If the game has got away from you completely, tell the arbiter rather than inventing moves that were never played.
There is one legal escape. If you have fewer than five minutes left in a period and no increment of thirty seconds or more per move, you are excused from recording at all (Art. 8.4), and the arbiter may keep score instead. When the scramble is over and you have time again, expect to be asked to bring the sheet up to date.
Keep the sheet afterwards. Typing your own game into an engine that evening and finding the move where it turned is the cheapest coaching available, and what you find is usually the same handful of blunders in the same handful of shapes rather than deep positional sins. If the writing itself is still slow, the fix is to get quicker at reading notation first.