Wash your berries well and put the Sierra gooseberries into a large pot. This seems like a lot, but they are large and very spiky, so they take up more space than other gooseberries. If you have regular gooseberries -- wild or domestic -- only use 5 to 6 cups. Barely cover the berries with water, cover the pot and bring to a boil.
Boil for 2 to 3 minutes. Turn off the heat and, using a potato masher, crush the berries to a pulp. Do not use a blender, food processor or immersion blender! If you do, you will merely make the nasty spikes smaller and harder to remove later.
Let this steep, covered, until it gets to room temperature, then pour everything through a fine-meshed strainer into a container and let it sit overnight in the fridge. The sediment at the bottom will be tan, the juice varying degrees of red or purple -- if you are using ripe gooseberries.
Strain again through a fine-meshed strainer with a piece of paper towel set inside. This will leave you with clear juice. This juice will store in the fridge for a week or so, as-is. Or you can heat the gooseberry juice with an equal volume of sugar and make gooseberry syrup. The syrup lasts months in the fridge. As for the sediment, taste it. If it is not too gritty, you can mix it with a thickener like tapioca and make it into a pie filling.
Note that prep time does not include steeping time.