Some extra tools I found to be invaluable in this whole process was canned air (used for keyboards and office dust), a microfiber cloth (preferably the one that came with your iMac if you still have it, but you can get these almost anywhere these days)and one of those soft foam monitor covers to put over the monitor while your friend is holding it up (a soft sheet or towel might work too). If your computer is a few years old like mine, there will be a considerable amount of dust that you will want to clear out (and will fly everywhere anyway). Once you cleared that out and replace the hard drive, wait until you've got the bezel and screws back on before using the canned air and microfiber cloth to make sure all dust and lint is clear of the monitor. This is where the second person will come in handy again. They can wipe while you spray. Get your glass top with suction cups still attached ready and wipe that with the cloth and air as well. While your friend does one last wipe of the glass and then the monitor, put the glass on as soon as possible so no more lint or dust falls onto the monitor. I did this and the monitor looks as beautiful and clear as the day I bought the iMac. It is truly a brilliant and beautiful design by Apple and LG.
Make sure you don't forget this step! I forgot this step, got everything back together, then had to take it all apart again to affix this! Not fun. That said, the second time went MUCH faster, and I found myself appreciating this design a lot more. It is much less daunting now, and is much easier than upgrading the RAM in a Mac Mini.
I found these instructions to be a little unclear. What you are looking at after removing the foam, is a bracket that is basically holding down the thermal sensor at the end of those wires coming out(which you can't see because it's UNDER the bracket). I found the bracket to be fairly easy to just pull up and off. The spudger (or even a tiny flathead screwdriver) can easily accomplish this if it's glued on tight. Once the bracket comes off, the thermal sensor is freed, and in my case fell away from the bracket. Just try to keep those pieces together in the order they came off. Fortunately, as easy as the bracket came off, it was easily pushed back onto the new hard drive with no trouble at all. The foam was pretty sticky still as well, although I had to double tape one side of it to secure it over the bracket.
I honestly didn't find the spudger to be necessary for most of this replacement. It is a nice little tool that makes one or two steps easier, but it is far from necessary. I imagine an old stylus you may have laying around would do just as good a job. I would definitely suggest putting the iMac on it's stand straight up to remove the hard drive bracket from it's post, as it sits very tightly in there. I found it popped out a LOT easier on it's stand than laying down, which was next to impossible. You do have to do some careful manuevering with your partner in order to accomplish this, but it's worth it. Take it slow.
As Marijn said, there are no disconnectable inverter cables on the 24" iMac, just a single thick cable in the middle of the back that is heavily taped with black electrical tape. I imagine you could remove that and retape, but it seems like it would be more trouble than it's worth. There is definitely enough clearance for someone else to hold the monitor up and away from the hard drive. For this reason, I would not attempt to replace the hard drive on a 24" without another person to help. The monitor is not heavy, and it doesn't take a ton of time to remove the old hard drive. It would also help if one of you has skinny fingers ;-).
I found this to be the most frustrating thing to remove out of everything under the hood. I had to have my wife use her skinny, smaller fingers to jimmy it up, but the tape kept pulling up and separating from the connector. I was worried that I was pulling in the wrong direction (what do I know about logic boards?) and that the tab was going to come off as it is slick tape. She eventually got it, but she did have to use some force. If you don't have a set of skinny fingers, just be patient and use a little force. I think the instructions should tell you to pull straight UP on the black tab, rather than 'away from the logic board', for those of us that don't really understand the technical components/guts of a computer. Honestly, my wife knows less than I do, but she could have done this as easily as me.
I am about as untechnically proficient as you can imagine, yet replacing the hard drive was not as daunting as it first seemed when following these FANTASTIC step by step instructions. I was forced to do this because of a lack of money, but having done it, I'm glad I did. Even if I had the money, it's not worth the $500 it costs to have an Apple dealer or Apple themselves replace the hard drive. Just take your time and don't force anything, and you should be done in no more than an hour. Kudos ifixit!
I would strongly suggest taping the screws down on a piece of paper towel or cloth in the same order you take them out, as there are different lengths in no particular order and they don't all fit into just any hole. Unless you keep track of which hole each screw belongs, there is no other way of knowing. I stress taping because I didn't tape mine down and after bumping the table I was working on they scattered, leaving me to guess.
Because there are no instructions for the 24", I used these and they worked fine. I shelled out $10 for the suction cups ifixit provides, but honestly the glass is really thin and lightweight, so cheapo suction cups from Home Depot should work just fine.