You bowl, if you get an open square then you just add that to the previous score, spare, 10+ the next roll and strike is 10+ the next two rolls.

For ex.

first you get a 2 and a 7: score right now is 9

then you knock down 9 and 0: your score is not 18

you get a 2 and 8(spare): right now you cant ad your score up cuz you need the next score to do so.

you get a 5 and a 4: so now, your score is 37. you add 10 and 5 and the 4(first roll of this turn)

you then get a strike: you cant add yet, need two rolls.

another strike: cant add yet since you only bowled once

you then get 9 and a 0: your score right now is... 65 (you add 10 another 10 for the strike and the 9. that gives you 52 you add the other 9 since you had an open square giving you 65)

yea, so spare = 10+1 more roll and strike = 10+2 more rolls

im not very good at explaining but i hope that helps.