I don't really know what's what either anymore!
The Nick Gammon tutorial seems very complicated, and this one which I found was simpler. For the Gammon tutorial, I didn't download anything from the Github, but I just copied and pasted one of the .ino sketches.
I am now trying an Arduino.cc tutorial here: Arduino Playground - MegaISP . It seems to be working a lot better, but when I try the command:
avrdude -p m328p -P /dev/cu.usbmodem621 -c avrisp -b 19200
I get: avrdude: stk500_recv(): programmer is not responding
I have installed Crosspack, as recommended in the Arduino tutorial. What has gone wrong?
I think that this one should do exactly what I want it to, in a simpler way than the Nick Gammon one.
UPDATE: I connected the 'heartbeat' LED to pin 9. The heart does indeed beat, but it freezes when I try to send any commands
UPDATE 2: The LED connected to pin 7 (the programmer LED) flashes quickly and briefly when I send the command, but nothing after that. The Pin 8 'error' LED shows nothing.