Very much simplified from a recipe by Martha Stewart that I'm not linking to because of her aggressive adblocker-blocker. For instance, she makes you wash the sugar that crystallises on the sides of the pan with a brush dipped in water, whereas I just push it down with the wooden spoon I'm stirring with if it annoys me.
It's easiest to use peel left over when you're making juice anyway. Otherwise --for instance if you want grapefruit pieces for a salad-- try to peel the fruit in as large pieces as possible. Scrub your fruit before peeling (that's easier than scrubbing the peel when it's off), even if it's organic: there may be dust or wax on it.
Pull any remaining pulp out of the peel (easy with oranges, harder with lemons). Use a sharp knife to loosen it. The peel doesn't have to be completely clean: you'll be scraping off the pith anyway.
After cleaning, put the pieces of peel in a large pan and cover with cold water. Bring to the boil, reduce the heat and simmer for 20 minutes. Drain and soak the peel in cold water until it's cool enough to handle.
Scrape the soft white pith off the peel with a melon baller (or a sharp spoon, or the back of a knife). Try not to tear or cut the peel. If your peel is too awkward to handle, you can cut it in largish pieces now.
Grapefruit peel needs another 20 minutes of boiling and another round of scraping, because the pith is very bitter and needs to come off completely.
Cut the peel into strips (anything from 2mm to 1cm wide according to your preference) with a sharp knife.
You'll end up with about as much peel as pith!
Heat water and sugar in equal volume parts in a thick-bottomed pan. You'll need enough syrup to generously cover the cut-up peel. If you're not confident about eyeballing it, cover the peel with water in the pan first, then pour everything through a sieve into a measuring jug so you can pour the water back into the pan and add the sugar to it.
When all sugar is dissolved, add the strips of peel to the syrup, and simmer until the strips are translucent and the syrup thickens perceptibly. Martha says about 40 minutes, but my batch of 9 oranges needed an hour and a half.
Fill jars with peel in syrup or fish the peel out of the syrup with a slotted spoon and spread it out on a plate to dry. For sugared peel, spread sugar on another plate and roll the strips in it before they're completely dry.
Any excess syrup is good on pancakes or to sweeten things that will benefit by a bit of orange flavour.