For folks who have the Apple platform, [color=rgb(40,40,40);font-family:helvetica, arial, sans-serif;]ND Timer, I don't know how to use it, but from what I read, it is pretty easy.[/color]
For Android, I found PHOforPHO, Phone tools for photographers, has a bunch of stuff but it also has most of the options as ND Timer, enter information, gives you the time and as a built in stop watch for the calculated times when you go into bulb mode. For this tool, you need to know your camera's circle of confusion; for DX it is 0.020, for FX it is 0.030. If you want other camera's go here for a table; http://dofmaster.com/digital_coc.html The COC is for other funtions, but needs to be set for the program. when you enter a shutter speed for this program, enter it as 1/125.
If you can't get any of these, then it is not that difficult to do the calculations on your phone if it has a calculator. Most have the option to have advance mode, as you will need to raise to the power. The main calculation is 2 raised to the power of ND filter, for a ten stop filter; 2^10=1024. You then take your shutter speed, example 1/125 and you get the decimal value for this; 1/125=0.008. Then multiply 0.008 * 1024 = 8 seconds. If all you have is a 10 stop filter, then I just gave you all you need. But, as Ron as indicated, it might not be 10 stops, it might be 11 or 9. But if all you have is the one filter and you know it's stops, then just need to calculate once.
The process is then;
1) Meter the scene without filter and set focus.
2) Calculate the new times.
3) Set camera to manual and manual AF and add filter.
3) Shoot with the new time values.
Of course, you will need a tripod. Need to shoot in manual mode. and need to be careful when adding the filter. Close the viewfinder.
But, you can use the camera meter (set to matrix) for step one, get the values for step one, and then switch to manual. As Ron said, using a light meter to get an average is the best, but if you don't have one, matrix does an average. You need to do some test shots to see how close matrix is, especially if your are at some extremes, like sunsets, sun rises and other like situations.
Another option is to use the camera's spot metering to take the average.
Please feel free to add to this or correct anything I have said.