This is a fast and easy way to generate the screen tone look digitally. It took me a bit of experimenting to figure it out. Hopefully some of you will find this useful.
Start with the initial line art.
Add the background
Add a white layer for the clipping mask so I can easily do a paint over.
I have the shading done on multiple layers, so in order for the next step to work I select all the shading layers, including the background, hide the line art layer, and choose the 'Copy Merged' option under 'Edit'. This allows me the get all the shading layers merged and copied.
Paste the new merged copy into a new window, change the Image type to Bitmap. If the option is grayed out, change it to gray scale f