[Sticky] Creating an artificial flat

Start by performing a calibration on the lights using a masterdark or masterbias. Press "create masters and assign to lights", after calibration save those lights (button all the way down in tab 2). Then clearing all data, load the calibrated lights again and take one to go through the "correct vignetting" tool in tab 9, in which you get to create boxes in the areas where there is the gradient. You can then check if that works. Check the "create artificial flat" and save that file. Then you should be able to assign the masterflat to the lights (check by switching to "l-calibrated" on top of the APP window).

Make sure your lights are not yet calibrated with a real flat. If that is the case, this tool won't work properly.
Of course, an artificial light will only correct for vignetting, not dust.