How do you double a recipe?

you just double the amount of the ingredients...Sometimes people will go into all kinds of theories about adjusting the baking powder but I've never found that doubling a recipe needs that kind of adjustment.

Oh... I double a bread recipe once and it got out of control. It didn't come out right at all.

You have to make sure your bowls and pans are the appropriate size.  If your mixer doesn't have the capacity for larger recipes, the ingredients won't incorporate properly and everything's going to splatter.  If you're doubling a bread recipe and using the same bowl to proof as you would a smaller recipe, it's going to spill over or constrict the rise.  If you double a cake recipe and fill the cake pan to the brim, it's going to overflow.  You might have to adjust baking times if you're going from an 8" cake pan to a 16" sheet pan (and this is probably where the baking powder theories fit in, but I think it only comes into play when you have very significant differences), but the recipe itself should be a straightforward multiplication of the ingredients.  Baking is all about ratios, and your ratios should stay the same as you increase the quantities.