Gather scraps from vegetables used in daily cooking. They can be stored in the fridge for a few days or in the freezer for a month.
Nice additions are herb ends, onion skins, tops of celery and carrots, any peels and scrapings. Do not use parts that are going bad. The skins do not need to be particularly cleaned as it will all be boiled.
To start I use a large pot with a tight fitting lid, add the scraps covered with water to about 2/3 of the height of the pot. Herbs, bay leaf, peppercorns, sage are all nice additions. Bring the pot to a boil, cover, turn down the heat so that it is boiling slowly for about ½ hour, then turn the heat to simmer for another 1-2 hours.
Let the stock cool, decant into jars and store in the fridge for a month or freeze. I like to use small jars with a wide mouth so that if I freeze it I can easily use the stock.
You can easily find recipes online or in excellent cook books with more extensive ingredients, etc. This is how I do it as a basic thing in the kitchen to use for soups, sauces and for fasting. Stock has the advantage that you don’t need much salt to make things taste great!
Enjoy and make it simple.