Rosa rugosa grows wild throughout Europe and the Americas - though it's not a native plant here, it does quite well. You can find them growing just as happily along a beach as in a field. They form a nice big bush, and have been traditionally used as hedge plants. The photo above is one I snapped in the djurgarden in Stockholm, Sweden - and it's nice because it gives one an idea of what the flowers and leaves of the plants look like, as well as the ripening fruits - just in case you want to go out and find some for yourself!
To make jam or jelly, gather a mass of the hips (bring along some shears and snip them off the plant) between now and October/November. I've read that they're better after a frost, because the cold will convert some of the starches to sugars, making the hips sweeter, but I often find that if you wait until October, the hips are spotted with blight, wrinkly, soft and generally not quite such fine specimens. It's up to you. You do want hips that are unblemished and firm - not wrinkly or soft.
When you get them in, wash the hips and tear off the green blossom-end. Snip any remaining stems close to the fruit.
You can make jam from these, but I prefer to make jelly, and here's why: if you make jam, you have to cut open the hips and remove the ring of seeds inside. Each seed is barbed with a little hair that itches like crazy. You must remove each of these seeds and hairs, because if they are ingested, they will irritate your digestive tract as much as they irritate the skin. So, for me, it's just easier to make jelly. There's more waste with jelly, but fewer hours spent picking through rosehips, trying to be absolutely certain that all hairs are gone.
So! To proceed with jelly, throw the hips in a pan and add enough water to cover by 1inch. Bring the hips to a boil, then reduce to a simmer and cook down into a thick mash. I find I have to keep adding water, and that this process takes several hours.
Once the hips are cooked down to a mash, they need to be strained for jelly. I made this batch before I had my food mill. Today, I would simply run the mash through the food mill and then strain the remaining lees through cheesecloth, but cheesecloth works. The only problem with using cheesecloth is that eventually, one's hands do tire, and there's always more liquid that could have been squeezed out. To strain through cheesecloth, set up a strainer over a large bowl. Lay 4 pieces of cheesecloth over the strainer, at different angles (so that the grids of threads in the cheesecloth overlap rather than aligning). Pour in the mash. Let stand 1- 2 hours (you can press on the mass with a spoon if you'd like, to help release liquids) to drain. Then, gather up the edges of the cheesecloth so that the mass is trapped in a ball inside. Twist the ends of the cheesecloth to tighten the cloth and add pressure to the mass, and squeeze the mass with your hands. You'll find that a lot more liquid will come out. Keep working the ball until you are tired, then compost it.
Then it's a simple matter of measuring how much fruit juice/puree resulted from the straining, putting the juice/puree back into the pan, and cooking it with sugar, a bit of lemon to taste, and pectin. I use a low-sugar pectin, so that I am not forced to make this jam too sweet, and simply follow the guidelines on the package regarding how much pectin to add to X quantity of fruit juice/puree. The rosehips are very high in vitamin C (in fact, during WWII, British factories produced rosehip syrup which was administered to British children to prevent scurvy and other diseases during the days of food rationing), so only add lemon if you like the taste. I do; I think it brightens it up a bit.
As far as processing goes, this jam is processed like any other: using the boiling water method, process for 10 minutes. For more information about canning and the boiling water method, and how long to process (it varies according to the altitude at which one cans), I recommend visiting the National Center for Home Food Preservation. It has all the official, goverment-approved information.
It's good to remember that this is a wild plant, and does not taste like a domestic plant. It tastes somewhat of apple (apples belong to the rose family) and somewhat of marmelade - Cass thinks it tastes like tomato. It definitely smells like marinara when the hips are cooking down into a mash. It's great with game, or used in thumbprint cookies at wintertime (especially if your thumbprint cookie dough has some ground nuts - almonds, walnuts, or hazelnuts - ground into it). I like to pair this with French chestnut paste to fill turnovers during the autumn and winter, too.