Growing birdhouse gourds is easy and turning them into bird houses is just as simple. However you have to be a bit patient and allow the gourds to dry, or cure, for a season, before you can begin the transformation from gourd to bird house.
To cure your gourds, you can simply leave them outside for the winter, in a somewhat sheltered site, or you can take matters into your own hands and cure the gourds indoors where they won't rot or get eaten.
If you didn't grow your own gourds, you're not excluded from the fun. Birdhouse gourds are available almost everywhere that ornamental gourds are sold. They may even have done some of the cleaning for you.