If it's not repeating:
rewrite the decimal number as a fraction (over 1), then multiply the numerator and the denominator by 10y (y being the number of decimal places it has), then simplify
If it's repeating:
name the decimal x, then multiply it by ten until it repeats the second time, the number should be 10yx (y being the number of numbers before it repeats the second time).
Subtract 10yx by x and simplify until you get a fraction.
0.654654654654... it keeps repeating "654" so we have to multiply it by 103.