In a perfect world, I would have done this before I had the frame powder coated, for feat of cracking the paint. But I forgot, and as it happens the powder coat was flexible enough that I could spread it without incident.
Perhaps this is obvious, but you should never try this with anything except an old steel bike.
The rear dropouts on this frame were 120 mm, ad the hub I intended to put in them is 135mm.
I followed Sheldon's directions for frame spreading, with a 2x and a stool.
You put the fork of the bike (no wheel or fenders on the ground facing away from the bench, stool etc.
You put one end of the 2x up on the bench and thread the other under the dropout and across the seat tube as shown. Then you push down on the end at the bike. Do this a couple of times and then flip it and do the other side a couple of times. Measure to see it it's too far or not far enough.
Next you need to check to make sure that the dropouts are still centered on the frame.
Sheldon suggests a simple test.
Run a string between the dropouts and up around the head tube.
Then measure the distance between the string and the seat tube:
This being one of the few things that's gone simply in this process, incredibly, I managed to get it right on the first try.
My next task, for which I didn't take any pictures, was I had to file down the front dropouts to receive the axle of the generator hub. Again, It would have been nicer to do this before powder coating, since this is a particularly vulnerable place to moisture. I will coat it with something before I'm done, I'm wondering if I can find car touchup paint in a matching color. If that doesn't work, I might just use black touchup paint.