Chop the carrots and celery into 1 or 2-inch chunks. Crush the garlic with a knife.
Add carrots, celery, onion, garlic, herbs, pepper and salt (if using), and vinegar to slow cooker.
Add water to slow cooker, enough to cover the vegetables.
Turn the slow cooker on low heat and cook for 8-10 hours.
After cooking, let the broth cool.
When broth is cool enough, filter your broth through a metal strainer so no pieces of vegetable, herbs or pepper make it into your broth.
Store in glass jar in the fridge for up to 5 days or it can keep in the freezer for several months for later use.