After finding the center of the barrel, I placed the copper sink upside down on the the top of the barrel and traced the outer flange. I then measured in 7/8" to allow a lip to secure the sink.
I made a pilot hole on the inside portion of the sink to get the jigsaw blade into to cut out the sink. You may have to make a few pilot holes if you are not able to turn the jigsaw at the corners of the sink opening.
I used a hole saw to make the opening for the faucet. With the single-lever style, I cut a 1 1/4" hole for the faucet.
A few boards towards the front of the sink were not glued well, so I secured them together with a couple of nail/staples as shown.
The barrel tops are usually glued well but this barrel was not as secure as I would have liked.