that's in the thousandths place so you would just put 258 over 1000.
258/1000. then it could be reduced since they are both even numbers, you would just have to find the greatest common factor (GCF).
more examples: for .2, it's in the 10ths place so put it over 10 (2/10)
or .25 is to the hundredths place so it would be 25/100