(Optional) Blanch poultry by submersing it in boiling water for 1-2 minutes. Discard water and rinse poultry clean.
Wash carrots and celery and trim off the ends. No need to peel the carrots.
Cut the top off of the onion and remove the outer layer.
Cut your produce into medium-sized chunks.
Separate the garlic bulb into smaller bits. This doesn't need to be clean or pretty.
Add everything to a large stock pot and cover with water.
Bring water to a boil, keeping an eye out for any scum that might be forming as the water starts to boil. Skim scum off with a wooden spoon or mesh strainer.
Simmer covered for at least 1 hour, preferably 2 hours. Add more water part way through if needed, but don't add any more water a half hour before you turn off the heat.
Let the stock cool for at least 20 minutes, but no longer than an hour.
Remove all of the solids with a fine mesh strainer and discard.
Run the last of the stock through a mesh strainer and portion out in whatever way you want to save it. This can be frozen for a few months.