I recently had to butcher a young 4 week old Cornish cross because the dog kennel panel I was putting up as part of their outdoor enclosure fell on it, fatally wounding it. Was too small for us to eat, so I butchered it for the dogs and as practice for me.
I cut its throat with a scalpel blade after breaking its neck, then hung it with a zip tie upside down to bleed out outside.
While it was finishing bleeding out, I got my supplies ready. I got a big stock pot with some water set to heat up with a little Dawn dish soap added in and some newspaper with my knife and kitchen shears.
I rinsed the bird off first in warm water to remove any obvious dirt and then dunked it in the scalding hot (but not boiling) water for about 30-45 seconds.
Next I started plucking, getting the wing and tail feathers relatively clean first since they are the hardest to pluck. I didn't worry too much about getting every feather off since it was going to the dogs and not to my table. In this photo, you can see the still very full crop on the bird's right side (the photo's left) towards the neck as a big bubble under the skin. Normally you would want to fast the birds for 12-24 hours before hand as it makes for a cleaner butchering process, but I didn't have the luxury of this.
Next I cut the head off with the knife and made a cut under the skin of the neck towards the crop, not wanting to break into it. I separated it out from the underlying tissue with my fingers. I went as far down into the base of the neck as I could and cut the crop out and threw it away. It was still a rather sludgy mix of water, feed, and a little grass.
The feet were then removed and it was time to make the cut into the abdominal cavity. Turn the bird on its breast and you look for the bird's cloaca or "vent" (the equivalent of a mammal's rectum) and hold up the left overs of the tail and cut about 2-3 cm below the cloaca using the shears to expose the abdominal cavity. Don't cut too deep because you don't want to cut into the intestines and make a mess.
Then after you make your two cuts, you reach in (I prefer wearing exam gloves, but many use their bare hands) and use your fingers to gently tease the guts away from their attachments and pull the whole thing (from esophagus to cloaca) out. Then you can reach in a bit further and find what feels like a firm grape and pull out the heart. The guts with the gizzard at the "front end" of the GI tract and the intestines at the "rear end" of the digestive tract are shown on the left and right side of the picture respectively. I already fed the liver and heart to the dogs, so those are not in the picture. I left in the lungs as again, it was just going to the dogs.
To butcher the rest of the carcass for the dogs, I use the shears to cut the breast straight up the middle and butterfly it as shown. The kidneys are shown with the two white arrows. Then I make a cut along each side of the neck and spine to make a separate piece. For large dogs, that's basically it.
So that's basically what I did. Obviously, you will probably want to make it a little prettier for table birds and pluck them much better, but that should help a little. Good luck.