I should note right up front that I'm bit of a techno-phob when it comes to doing hardware upgrades/installs, which I blame on the time I practically blew up a NuBus card on a Mac II.
Apple provides detailed instructions that recommend lying the iMac down on a towel and attacking the memory slot from the side. That might work on a table, but after clearing the floor, I found I did most of the installation with the computer resting on its top edge.
The next problem I encountered was that the Philips head screwdriver I had didn't fit the screws! A quick trip to the hardware store solved that problem.
The screws are short and only have to be turned a two or three times (above). After removing the cover, you pull out the two plastic ribbons (below).
If you're replacing the original memory cards, you pull up the plastic ribbons to release the cards (you have to give them a good tug!)
Then you put the news cards in, making sure you put them in the right way. Again, it takes a bit of force to push the cards in; they don't quite click into place so much as "clunk"
Then replace the memory cover and restart. Ta dah! The computer made the happy start-up sound and came up with 16GB first time. I haven't always been that lucky.
If you don't count the trip to the hardware store, it probably took less than five minutes. YMMV.
Apple Support: iMac: How to remove or install memory