Hi Aimee,
Great question. Usually with rosehip oil it is actually the pressed oil from the rosehip seeds.
Saying that however you can do an infused oil with the whole hips (including the seeds) and still get wonderful nourishing, anti-oxidant and anti-inflammatory effects from it.
You can harvest the whole hips, clean them up (leaves, twigs etc), then mash them up a little bit so that the seeds are exposed and bruised a little as well (to allow the goods to infuse out of the seeds and into the oil). A mortar and pestle or back of a spoon in a bowl could do this. Use the warm method, then afterwards strain through multiple layers of cheese cloth or some muslin. If there is still some ‘debris’ after straining, you could let the oil sit for 24 hours and decant off the clear oil.
I hope this helps, and you enjoy creating with rosehips!
Benna