Practice unit conversion. When you have to convert bigger unit into smaller then you multiply and when you have to convert smaller digit into bigger than you divide.
1 kilogram = 1000 gm so 1 gm will be 0.001 kilogram.
1 liter = 1000 ml so 1 ml = 0.001 liter.
So when you are converting gms to kilograms then put a decimal leaving last 3 digits and when you are converting kilograms to grams then put three zeros at the end of the number or multiply the number by 1000. Same is the case with liter-mililiter conversion.
