Toss the onions in the fat (melted, if the fat is solid at room temperature.)
Cook the onions in a heavy pan over a low heat for 45-60 minutes, stirring every 5-10 minutes.
Remove the onions when done, then add the liquid to the pan. Scrape up the browned bits and liquid and add them into the cooked onions. Use salt and pepper to taste.
Cast iron pans are an excellent choice for caramelizing onions because they retain the heat while minimizing burning. Heavy gauge stainless steel also works fine. Avoid light pans, which may not prevent burning and those with no-stick coating, which inhibit browning and caramelization.
Onions sizes can be tricky. I prefer to weigh my onions. One-pound of onions, sliced and cooked down ends up to be about 3/4 cup of caramelized onions.