Stock. It all begins with homemade stock. You can use chicken or beef, or any meat-based stock. For the bouillon above I used pork stock for a delicious rich, bacon-scented flavour. It's pretty awesome.
Skim. I like to cool my finished stock overnight, then skim off any fat that has risen and congealed. Then I place it in a pot and begin the reduction. Don't worry, you'll still get plenty of flavour, just none of the grease.
Simmer. Simply reducing stock to a syrup-like sauce is all it takes to make homemade bouillon. I boil it fast at first, and then lower to a simmer when it is about half reduced. Why? Because it can get away on you towards the end, over-reduce and possibly burn.
Sauce. On average, a gallon of stock will reduce to about 1 cup of liquid bouillon, but rather than measure everything (which I almost never do), it's better to know what to look for in a reduction of stock. It will have thickened into a lush sauce. It should coat the back of a spoon, and hang out there, all glossy and rich. It's about the consistency of hot maple syrup, still runny, but with some weight to it. Now cool the sauce!
Set. Line a pan (loaf pan, 8x8, 9x13.....depending on how much stock you boiled, you can probably eyeball this) with plastic wrap. Pour the cooled sauce into the pan and place in the refrigerator overnight. The sauce will set like jello.
Slice. Invert the brown jello onto a board, peel off plastic wrap and cut into squares. Package in ziplock baggies or small jars. Store in the refrigerator for up to 2 months or the freezer for up to 1 year, but they never last that long.