Can you turn all-purpose flour into self-rising flour?

I understand that self-rising flour is just all-purpose flour that has had some extra ingredients added to it. But I can’t remember what the extra ingredient(s) is/are. Baking powder? Anything else?

The reason I ask is that I usually do not keep self-rising flour around, as it seems that all-purpose is more useful. (If some baker jellies disagree, please let me know!). But if a recipe calls for self-rising flour, it would be nice to be able to make some on the spot from all-purpose flour.