First, measure exactly how many nanometers you need to calculate into millimeters.
Get a calculator or a pen and paper and begin processing the calculations. Take the number of nanometers, then divide them by 1,000,000 (x/1,000,000 = y) to get the number of millimeters. To convert to nanometers, take the millimeter amount and multiple by 1,000,000 to get the number of nanometers (y*1,000,000 = x).
Run the calculations again to be sure that your work is accurate and precise.