My line about using the machine "properly" is more in regards to safe operation of the Harley or the Corvette, because it's literally possible to kill someone if you use them improperly, which would obviously be far more harmful than a PPC Mac in a botnet.
However, some practices with the Mac that would be using them "properly" would be avoiding untrusted software installation, using strong passwords/passphrases, keeping the machine behind a firewall, and installing the latest software updates. Even following those procedures, though, partially because there are no more software updates for PPC Macs, combined with the huge attack surface of OS X, you are at extreme risk for your computer's resources being used against the internet by malicious actors.
Using an insecure Unix-based platform on the internet isn't like using an old Jaguar Mk2 on the road, it's like putting a gun safe with a broken lock outside, and storing firearms and ammunition in it, and expecting that someone won't use them in a crime.
However, some practices with the Mac that would be using them "properly" would be avoiding untrusted software installation, using strong passwords/passphrases, keeping the machine behind a firewall, and installing the latest software updates. Even following those procedures, though, partially because there are no more software updates for PPC Macs, combined with the huge attack surface of OS X, you are at extreme risk for your computer's resources being used against the internet by malicious actors.
Using an insecure Unix-based platform on the internet isn't like using an old Jaguar Mk2 on the road, it's like putting a gun safe with a broken lock outside, and storing firearms and ammunition in it, and expecting that someone won't use them in a crime.