My guidance for this comes from a variety of sources, but mainly from Pam Corbin's River Cottage Handbook No.2 Preserves. Truly a favorite for me.
Combine 1 kg roughly chopped (cut out bad spots, scabs and bugs but do not peel or core) sour cooking apples with 600 ml water. Try also redcurrants, crabapples, gooseberries or quince.
Bring to a simmer and cook gently for 45 minutes to 1 hour, or until the fruit is soft.
Strain through a jelly bag, muslin or clean pillow case (Note: before using a jelly bag, always scald the bag first in boiling water. Bags, though stained, may be reused after washing and scalding). I know it's tempting, but don't squeeze the bag if you are making jelly that you wish to be clear. Squeezing causes cloudy jelly - if clarity doesn't matter, by all means, squeeze away.
The resulting pectin stock can be kept for up to 4 weeks in the fridge, frozen or sealed in sterilized jars. It will diminish in potency so allow for a reduction in strength when using stock that has been frozen or sterilized. I seal mine in sterilized jars and finish in a boiling water bath. Be sure to store in a dark, cool place as the stock can become discolored.
To use the stock, stir 150 - 300 ml of it into every 1k of low pectin, softened fruit before sugar is added.
An alternative: check out this post on Lindy's Toast - I loved Lindy's article about pectin stock and the accompanying recipe.