This is all largely guess work. My oranges came from a tree in the backyard so I didn’t measure or weigh much at all. The oranges were too bitter to vere function as dessert oranges but were perfect for marmalade.
A little theory.
What sets marmalades is the pectin in the pith and seeds of the fruit in conjunction with the vast amounts of sugar used to create the syrup. Originally I did the traditional marmalade with the pith left on the peel and a muslim bag for the seeds to cook in. However I don’t like the pith left on the peel I prefer just the fruit zest and I like my marmalade with much less sugar than most recipes recommend (usually they say the same weight as the fruit or even more). What happens if you reduce the pith content and the sugar content is you have to cook it for longer to get it to set. Overcooking is death to marmalade. So all I did was use packets of pure pectin which is sold as jam (jelly in the US) setter for use with pectinless fruit.
What I do.
Have your packet pectin on hand so you can use it when necessary.
Grab say a dozen decent size oranges and using a grater strip off all the zest leaving the pith. You want nice fine shreds.
Do the same to a lemon (and anything else you fancy some mandarins, a grapefruit or any citrus). Keep a note of what you put in because some mixs are really good just because the local fruit is good. It’s your own recipe then.
What you are meant to do now is halve the fruit and squeeze the juice so that you get nice pulpy fruit juice without seeds and membranes.
In a large non-reactive pot (not aluminium) start with 2 cups of water and the fruit zest and simmer for 10 to 15 minutes before adding the juice. Hopefully you’ve got about 3 cups of zest and 5 or 6 cups of juice. If the fruit isn’t very juicy I have resorted to adding pulpy commercial OJ with no bad effects. Simmer away gently until the zest is very soft (it won’t taste good yet though). Keep adding water as necessary to stop the mixture burning. Most recipes call for pints of water but since most of it evaporates off I think it’s only so you can wander off while it simmers.
It may take 40 minutes for the zest to soften (or even more I guess I only know my fruits) but when it does it’s time to add the sugar. Normally recipes call for equal amounts of sugar up to twice as much sugar. I start with about half (by eye just pour it in) and sometimes I use honey as well for a slightly different taste. You want to boil this mixture for about 15 minutes stirring constantly. Keep a cold plate nearby so that you can chuck a dollop on there, let it cool and then taste it so you can adjust the sugar to your own taste. When the mixture is too your satisfaction take it off the heart and follow the instructions on the pectin packet to set it. Because I don’t add lots of fluid I don’t usually need much pectin. It’s thick enough if it kind of sets on your cold plate. You can thicken it by more pectin or more boiling.
Now for bottling I am slack. I know you are meant to be careful bottling home produce but all I do is boil the jars, make sure they are filled to the brim and seal them with screw top lids. I just use used jars I’ve gathered over the year. I tell people to use the marmalade within 3 weeks (arbitrarily arrived at). For the Southern Comfort marmalade what I do is pour a shot of SC into the jar, swirl it around pour it into the little jug I use for bottling. I then fill the jug with marmalade, mix in the SC and stick it in the bottle and seal. The completed bottles then get boiled again and put aside to cool.
I hope this helps. It’s not as complicated as I’ve made it seem but it is real hands on cooking.
I’d recommend that you have a trial go with just a few fruits and make a little batch and see if you have any problems or make any discoveries.
When you make the marmalade you can recover from most stuffups, although many are eliminated by using commercial pectin (the pectin content of fruit is very variable and as I said overcooking is death). Even if you finish a batch and it’s too thick or too thin, you can just make a new batch and throw the old batch in to remedy the problem.
Don’t use a food processor to get the juice it makes the marmalade look crappy. You want nice clean juice with only pulp and zest, better to augment with commercial OJ than try to increase the yield from your fruit.
A last thought that occurs to me is that what I was trying to make was “better” versions of Robertson’s English marmalades which may not be to your tastes.