While I am cooking, I set aside my veggie scraps in a large mason jar in my freezer. If it sounds like it would make a good broth, go for it! Things like carrot peelings/ends, asparagus ends, broccoli stems, onion peels/ends, garlic skins, leek stalks, kale/chard stems all make great broth. I tend to not put bell pepper in mine as it made it bitter once and I was sad :( but people say this about broccoli and asparagus and i have never had an issue, so I think it comes down to personal preference.
Once the jar is full, I toss the scraps into my slow cooker, add a tbsp of salt, cover it in water, and cook on low for 8 hours. I then let it cool and squeeze all of the broth out of the veggie scraps before adding them to my compost. I store it in jars in the fridge, or put it in straight pint mason jars in the freezer. These can easily be pulled out and defrosted when needed. If I run out and need some ASAP I will put everything into a pasta pot and get it boiling on the stove, and then simmer for a few hours.
So there you have it- free and zero waste veggie broth! This has saved us soooo much money and waste, plus it gives me peace of mind knowing exactly what my veggie broth contains.