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Nine sheets, each one a little harder. Work out the running total and the answer shows you the arithmetic either way. By the end you can score a game without a screen.
A strike is worth ten plus your next two balls. Not the next frame — the next two balls. When the frame after a strike is also a strike, that frame only supplies one ball, so the bonus reaches into the frame after that as well. This is where almost every scoring mistake comes from, and it is why three strikes in a row is worth so much more than three spares.
Ten plus your next single ball. One ball, always, no matter what that ball is. A spare followed by a strike is worth 20 — not 20 plus the strike's own bonus, which belongs to the strike's frame.
Twelve strikes, not ten. Frames one through nine are each worth 30, and the tenth pays 30 for its three balls. Nine thirties plus thirty is 300. The last two balls exist to settle the bonuses on the ninth and tenth frames rather than being frames of their own.
Score forward and keep a running total, rather than working out each frame in isolation and adding at the end. When you hit a strike, look ahead two balls; for a spare, one. If your total ever passes 300 you have counted a bonus twice, which is far and away the most common error.
If you want a tool rather than a test, the score calculator fills in a full sheet as you type it.