Tap on the little crafting icon along the bottom of the screen. This looks like a bird holding a hammer. Below the heading there are 3 options and you need to tap on the first one which is a little bird in a nest icon. You can select a nest type, depending on your level, and it will show the required items. If you have enough you can select "craft" or there is an option to pay with birdbucks to get the necessary items.