I’ll have to begin with bones, leftovers, and water. The scraps are all the trimmings from the veggies and meat I’ve recently cooked. Every time I cook, I put the scraps in a freezer container. I buy a few of meaty bones when I’m ready to prepare soup. They’re really inexpensive, and I occasionally acquire them for free as butcher shop rejects.
In a stock pot, I put the leftovers and bones. I soaked them thoroughly in water. Bring to a boil, then reduce to a low heat, cover, and leave to simmer for a bit.
You can add onions, salt, pepper, or anything else you like, but those are the fundamentals. Unless I’m making a potato soup or anything similar. I’m going against the grain and not using chicken stock. For the “broth,” I make a blond roux and add cream. I do the same thing with broccoli soup.