12.023 is the same thing as saying 12.023/1, correct? This is technically fraction form, but we need to simplify it further to get rid of the decimal point in the numerator.
If we were to multiply 1,000 to both the numerator and denominator, we would still get the same answer: 12,023/1000. Multiplying both sides by 1,000 essentially means to move the decimal point three places to the right, thereby giving us 12,023 instead of 12.023. As for the denominator, if the decimal point is already at the end of the number, you just add in the amount of zeros that were in the number you multipled it with, which in this case, is three. So you'll get 1,000 for the denominator.
Technically, some teachers will accept this as the most simplified form, but others will ask for a mixed fraction answer, in which a regular number is combined with a fraction. If we were to divide 12,023 by 1,000, we would have 12 with some remainder, which is estimated at 23/1000. So your mixed fraction answer would be 12 + 23/1000.