The 3 inch wood screws drive in just about okay with my 6.7 amp drill. I drive the screws in tight so the MDF panels will eventually rest flat over where the screw-heads are. Altogether I use 7 screws, plus the two nails. They're not all in a straight line, as I think I read in a home improvement book somewhere that you should stagger them slightly to get a better grip. So now the first of my 2 x 4 x 90 furring planks is up.
Hollow sound means space, solid sound means stud.
Gotta try that one next time. The book also suggests that my design is a bit unconventional. I am learning so much from this project. So let's try the same technique with the next 2 x 4 x 90. Again, I pin it in place, top and bottom, with 2 inch bright common nails.