I did this around 3 years ago. There's a problem when connecting the new hard drive with the hard drive cable. I'm using a 500GB WD Blue and there seems to be a "tooth" on the iMac connector that won't fit the WD Blue. The only solution was to file off that small plastic tooth. I also didn't need suction cups to remove the glass panel. I accidentally discovered another way, when I was running my finger nail around the seams, it popped up. I wouldn't suggest it though.
In step 14, the Apple connector for the hard drive has a locking mechanise that doesn't fit most replacement hard drives. I got an identical hard drive that doesn't have a provision for the connector. A little bit of shaving or filing of the plastic connector did the trick.