That would depend on if you are asking about making stock or canning stock.
If you're just making stock, you can certainly just boil up some bones. If you're canning the stock, you must process it in a pressure caner; the pH isn't acidic enough to kill botulism.
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Very few people here in Australia would use the Pressure cooker method. A pot and hot water in all you need to start your stock.
A pressure cooker just speeds the cooking up. You can follow the recipe in this beef stock Instructable, but just let the ingredients simmer for a few hours. https://www.instructables.com/id/Pressure-Cooker-Beef-Stock/
My wife and mother-in-law both make chicken and turkey stock by dumping the bones into a pot of water and letting them cook away for a few hours. It seems to turn out deliciously.