There doesn't seem to be a good document out there on how to use the NanoDLP mask generation wizard someone going to do a quick one that at least provides an overview.
The first thing you need is a meter that can read the UV light coming from the printer. There are number of options discussed elsewhere on this blog. You can whip one up using a voltmeter, (with a battery, a resistor, and a photoresistor), you could use one based on an Arduino microprocessor (with a resistor and a photoresistor), or you could use a commercial device. As the readings are relative to each other it doesn't really matter what you use!
You also need to understand the screen orientation as represented by NanoDLP relative to the actual printer.
This article should help.
Of course you should also understand conceptually what you are trying to do! A mask is a grayscale image that evens out exposure from the less bright areas to the bright center. The mask will be clear at the edges where the light doesn't reach as well as the center where it will be dark. The wizard lets you measure points on the screen and adjust them so that they are all at the same level of brightness. It then generates a mask based on those readings.
As this is just an overview I'm not going to talk about the details of creating a mask as you should be able to figure that out by messing with the wizard. In general what you need to do is display the grid of cells of the size that you will use. Find the one that is the dimmest, usually one of the corners. Adjust all the other cells to the same reading as that dim cell. Generate the mask!
I believe the new is diversion of the wizard allows you to export a CSV file that represents the mask. My D7 is currently in pieces being upgraded so I am not sure of this. If indeed it does you should save one of these exports! I would also recommend getting the image file that is generated by the wizard from the RPI and saving it on your workstation. You can then upload it at your leisure and use it only when it is needed as opposed to all the time with longer exposure that goes with it.