First, take three off it, because that will form the whole-number part:
3.92 = 3 + 0.92
Next, read off the place value of the last digit. The 2 is in the 'hundredths' column, which means that 0.92 = 92/100:
3.92 = 3 + 92/100
Finally, simplify 92/100 by dividing top and bottom by 4 to get 23/25. Then, shove it all together:
3.92 = 3 23/25