For every cup of flour you need two cups of sugar...
1 cup flour to 2 cups sugar.
posted by Ms Pi_3.14159265358979
To add to Ms Pi's example,
ratios can have more than 2 terms.
e.g. to make concrete, you should mix
water : cement : gravel (sand) = 1 : 2 : 3
in words: use 1 part water with 2 parts cement and 3 parts sand.posted by Reiny