The basic method is to take a pectin rich fruit and extract the water soluble pectin by boiling it, then concentrating the liquid. Apples are a good source of pectin because they both contain a lot of pectin and have a relatively neutral flavour (as opposed to orange peels for instance).
Step 1: Ingredients and tools
3 lb Apples
4 cups Water
2 Tbsp Lemon Juice
Jelly bag/cheese cloth and strainer
I am using the basic method from www.pickyourown.org, a website with a lot of very helpful information.
Step 2: Cook Apples
Strain the results through a jelly bag to extract only the liquid. If you are concerned about cloudiness don't squeeze the bag. If you don't have a jelly bag (though I highly recommend getting one if you plan on making jelly) you can line a strainer with a few layers of cheese cloth.
Step 3: Reduce Liquid
If you want to test the level of pectin in your reduction, put a few drops into rubbing alcohol and the pectin will gel almost instantly. Obviously this should not be eaten.
Step 4: Store and Use
Step 5: Example of Use
First I extracted the juice from the rhubarb by boiling and straining it.
I added a few cubes of pectin and tested the pectin level using a method that I found in Joy of Cooking: All About Canning. To test the amount of pectin I placed 1 teaspoon of juice in 1 tablespoon of rubbing alcohol then fished out the congealed pectin with a fork. A low pectin mixture forms loose strands of pectin while a high pectin mixture will form a single gooey ball.
Once the pectin level was looking good I carried on as usual, adding sugar and cooking the mixture until it passed the jelling test.
Remember that it can take 1 to 3 weeks for jams and jellies to set.