OK so it was late at night when I was making this and frankly I took crap pictures. I think you get the idea though :)
Every time I use any veggies I save the peels, ends and even onion shuck. I throw it all in a ziploc in the freezer. A gallon size is just about right for making broth. When it's full I simply dump it in the crock pot and let it run for 4-6 hours until the veggies are mush and the water is golden brown.