Label a gallon freezer bag. Every time you make a dish and have vegetable scraps, add them to bag.
When bag is full, add vegetable scraps to 6 qt. crock pot or a dutch oven. Brussels sprouts stems, onion and garlic skins and ends and sweet pepper stems and seeds.
Next, I usually clean out the vegetable drawer of any older vegetables. Asparagus ends, 1/2 of an onion that was old (but not rotted) and some baby carrots that have seen better days.
Add any herbs that you have in the vegetable drawer, as well. Here I added thyme and sage.
Add water just to barely cover. Cook on low for 5 or 6 hours or on high for 1 to 2 hours.
Strain solids from the stock through a colander. Discard scraps. Use right away or place in freezer bags and freeze. The stock will freeze for many months.
Notes
Keep a large freezer bag labelled in your freezer. As you are making other dishes, save all of the vegetable scraps you have. Vegetables that work well for vegetable stock are skin, leaves and ends of;
onion
garlic
cauliflower
brussels sprouts
broccoli
carrots
celery
parsnips
squashes
Add smaller quantities of stems of herbs, such as;