When putting together your Proto or Doubler Featherwings, think about how you want it to connect, you can use stacking headers:

Or plain female socket headers:

The most common method of attachment for the Proto is putting stacking or female headers on the Featherwing and then putting the Proto on top:

But don't forget, you can also put the stacking headers on the Proto wing and stack the Feather on top of it!

For the Doubler, you can use it side-by-side

By soldering female or stacking headers onto the doubler, but you could also have mulitple stacking wings.