Put the white beans to soak in water overnight in enough water to cover them by several inches. They will double in size, and you should end up with around 4 cups of soaked beans.
Strain the water off, and rinse the beans, and add them to your pressure cooker with around 4 cups of your stock or broth of choice. You should have the level of liquid above the level of the beans. If not, add a little water to your pot.
If you will be making this on the stove, a lot of the water will evaporate. When I made it on the stove, I suggested having around 7 cups of liquid (stock plus water) to compensate for that fact. Just make sure you have enough liquid to cover the beans, and add in water as needed to keep them covered.
If using the pressure cooker, use the setting that it has for legumes, or the one for soup. On my pressure cooker the legumes setting lasts around 35 minutes once the heat level has been reached, and the soup setting lasts for 30 minutes. We will be cooking the beans with the chorizo and spinach for a couple of minutes after they are finished cooking, so you may choose to cook them less time if you like the beans having a more solid consistency.
If using a pot on the stovetop, simmer for around an hour and a half and check the beans for tenderness. Add in extra water as needed to maintain the desired consistency.
As the beans are finishing up, I like to slice the chorizo and pan fry it in a medium to large pan on the stovetop. I then pour off some of the released fat as I have found that it only forms a layer of grease on the soup, and doesn't really add more flavor. Once the beans are finished, I add them to the pot and simmer it for a few minutes with the chorizo to incorporate its flavors into the soup.
If making on the stovetop, I don't usually bother using another pan and just add the chorizo directly to the beans on the stovetop as soon as the beans are mostly tender. I then let them simmer together for 5-10 minutes more to allow the chorizo to cook and add flavor to the soup.
Shortly before serving, add a few handfuls of spinach to the soup. Let it wilt and add some more if you like.
Taste the soup and add seasonings (salt, pepper and paprika) to taste.
(October 2015 update: I'm adjusting this recipe for making with a pressure cooker as it is quicker and easier and uses less broth. I'll explain how to make it in a pot on the stove, though, too.)