Stiff, flat brush (I've used 242 and 252). No base. Pat on a thin layer, then use a window wiper movement to spread evenly. Let dry and then pat on an extra layer, if more intensity is wanted. To shade up into the brow, besides patting on the lid, I'd also pat a small amount just in the center of the brow bone or at the corner, depending on where I wanted the most intensity, then window wiper it out quickly.
I also tried some rounded blending brushes, but those weren't quite as easy to control - they were doable, but only if I was using extra light pressure. I'd say a brush like that would be better if you wanted these shadows extra sheer.