How do you write 648 thousandths?

The thousandths place is 3 after the decimal place. That is, 1 thousandth would be 0.001

To write any number of thousandths, you put the unit of that number in the thousandths place, the tens of the number in the tenths place, and so on. In this case, we want to write 648 thousandths, so that would be written as 0.648