I tried using the sonoff DIY tool. It would never connect or give it would give me an error. So I bought some Pogo pins and a piece of wood and did a little soldering. see the attached picture of my fixture. Flashed the first sonoff mini without any issues.
I use an old sonoff mini and drilled some holes through the pads to locate the position of the Pogo pins. See the photo. The Pogo pins on the outside of the board help position it. Rubber band hold it down while flashing.