A paper scorecard has two halves and three totals. Filling it in the same order every time makes mistakes obvious rather than invisible.
Write the score the moment the turn ends
Decide the box, write the number, then pass the dice. Leaving a turn unrecorded while the next player rolls is how boxes get filled twice.
If you are playing here rather than on paper, the card does this for you and will not let a used box be chosen again.
Keep the upper section together
The first six boxes are Ones through Sixes and they only count dice showing that number. Total them before you touch the lower half.
If that subtotal reaches 63 you add 35. Check it once when the upper half is full rather than recalculating it every turn.
Three of a kind and four of a kind take the whole roll
Both boxes score every die, not just the matching ones. This is the single most common scoring error on paper.
Four sixes and a one scores 25 in either box, not 24.
The fixed boxes never vary
Full house is 25, small straight is 30, large straight is 40 and five of a kind is 50, whatever the dice add up to. Writing the dice total in one of these boxes is the second most common error.
Check the two totals against each other
Add the upper half with its bonus, add the lower half, then add those two together. Doing the sum twice in the same direction repeats the same mistake, so add the second one from the bottom up.
A finished card for a reasonable game usually lands somewhere between 150 and 250. A total far outside that is worth rechecking before anyone celebrates.