Gather your slow cooker, chopping board, knife, measuring spoons, and ladle. Make sure everything is clean and ready to go.
Chop the carrots into thick slices, about half an inch each, and finely dice the celery and onion. Mince the garlic if you're using it, and shred your cooked chicken into bite-sized pieces.
Layer the carrots, celery, and onions at the bottom of the slow cooker. Spread the shredded chicken evenly on top. Pour in the chicken broth, making sure it covers the ingredients by about an inch.
Add salt, pepper, and your dried herbs or spices. Give everything a gentle stir to distribute the seasonings evenly, then set the slow cooker to low and cook for 6 to 8 hours, or on high for 3 to 4 hours.
As the soup cooks, it will fill your kitchen with a warm, savory aroma. Check occasionally to ensure the broth is simmering gently and not boiling vigorously. If needed, add a splash more broth to maintain the level.
About 15 minutes before serving, stir in chopped fresh parsley or thyme for a bright, herbal finish. Taste the broth and adjust salt and pepper if necessary. The vegetables should be tender, and the chicken warmed through.
Turn off the slow cooker and let the soup rest for a few minutes. Use a ladle to serve hot into bowls, garnishing with extra herbs if desired. Enjoy the comforting aroma and hearty texture of this simple homemade soup.