I've slavishly followed Delia's recipe for a few years and (even for an untalented cook like myself), it really works.
https://www.deliaonline.com/recipes/type-of-dish/preserves/traditional-seville-orange-marmalade
Mine is rather simpler, but with same proportions. I cheat by adding pectin 'cos I can't be arsed with the pip malarky:
2kg sugar per 1kg oranges.
Pectin per instructions on packet (one sachet pectin per kg sugar for mine)
1) Simmer whole oranges for 90 minutes in enough water so they just float.
2) Halve them, scoop out pips and bin those.
3) Chop skins however you like them, then return to water, add sugar and pectin, boil until set.
My preferred setting test is to put a little bit on a chilled plate, leave for a minute and see if a skin forms when running your finger through it.
That's it.