This app was mentioned in 1 comment, with an average of 8.00 upvotes
You can't see anything through the viewfinder with the filter on, so first you expose normally without the filter attached. You make a note of your settings, and then attach the filter. To get the new exposure time that you need to use with the filter attached, it's easiest to just use a smartphone app. I usually use this one. You can also carry a chart, or ballpark the calculation in your head. Either way, you'll probably have to play with the timing a bit to get results that you're happy with.