To convert decimals to fractions first you should remember that all decimal places have a base-ten place value. So, the first place behind the decimal point is the tenths, the second place is the hundredths, the third place is the thousandths, and so on.
If you are given a decimal such as 0.312, simply say the decimal in the proper notation. ("three hundred twelve thousandths"). You should be able to 'hear' the denominator of your fraction. Therefore the fraction form is 312/1000.
You must always reduce your fractions to their simplest form. Since the GCF of 312 and 1000 is 8, take 312/1000 divide your numerator and denominator by 8/8 to get 39/125. This is the fraction form of the decimal 0.312.