Because no-one wants to waste money these days, and also because it's so damn tasty, I always try to make my own stock whenever I can. That said, I use stock cubes more often than I use home made stock, mainly because it's more convenient, and I don't always have stock in (although I do freeze it when I can, for just such times)
I prefer to use a slow cooker for my stock, although just leaving it on a low heat for a long time works just as well. Like anything when it comes to cooking, my approach is that it's not an exact science, and every time I make it, there are different items in my stock, depending on what's to hand.
Typically I will use carrots, celery, onions and a leek for the vegetables, thyme, bay leaves and rosemary for the herbs, garlic cloves (crushed) and lots of seasoning. This is added to the bones, cartilage and connective tissues (I love that term, although I know it makes some people a bit squeamish!), skin and meat, then just simmer or slow cook for as long as you are able. This is where slow cookers have the advantage, as they can be left on over night
Once its done, strain the bits out and that's your stock! Ready to be used or frozen. It helps to skim off the fatty layer that collects on the top as well. I haven't given any exact measurements, because, like I said earlier, it's up to you what you do, making stock, so just play around and see what works!