My 11 year old and I followed this detailed guide and it worked like a charm. Two sets of eyes were helpful, but Jr. really did this almost entirely on his own. Once finished (just before re-sealing the iMac) we followed a youtube video to create a “bootable USB thumbdrive” to start-up the refreshed iMac. We then reinstalled the operating system (from the internet) onto the freshly installed hard drive. Very happy and proud. Very grateful for this well written guide.