Count the digits from the right of the decimal point. There are 4. Now write 1 followed by this many zeroes: 10000. This is the denominator.
Take the decimal and remove the decimal point and any leading zeroes and write it as the numerator over the denominator: 681/10000. That's your fraction.
Sometimes it will cancel down, for example, when you can divide top and bottom by a common factor like 2, 4, 5, etc. 681 and 10,000 don't have any common factors.