How to make an egg shell mosaic bird house gourd.

1.  The gourd.  
Either grow, or buy a gourd.  They come in many shapes and sizes now. The bigger it is, the easier it is to work with. (And the more space you will have to cover with shells!)

A fresh gourd will need to completely dry out.  Put it in a cool, dry place.  Like a garage IF it stays above freezing.  And leave it alone.  Make sure if you have many gourds they are not touching.  They tend to rot.
The gourds will get moldy.  This is ok.  If it really bothers you, you can wipe them down with a mild bleach solution but it really isn't necessary.
When you can shake a gourd, and hear seeds rattling around in there, it's dry.

Now is the time to really clean them.  I mix up a big bucket of mild bleach water. Get a scrub brush and scrub the gourds down.  You will remove all the fuzz, parts of skin will also sluff off.  The gourd is left with a mottled surface.  (Actually quite attractive just covered with a clear coat as is.)  Rinse and let dry.

2.  Cutting.
Sketch a pattern on your gourd.  Start easy.  Simple shapes work well.
I started doing these with a plain old jig saw.  Drill a hole so you can get the blade in and go at it. Let the saw do the work. Don't push too hard.
Later I bought a dremel power tool which allows for a much more controlled, neater cut.
What ever works for you.  Even a small hand saw will work.
Sand or grind the edges of your holes.

Once you have an opening, you'll want to remove the innards.  (Save the seeds if you have room to grow your own.)  Scrape out all the stuff inside.

For true bird houses, depending on the bird you want to attract, they all require a specific size entrance hole.  Look up birds on the net for info on that.  By the way, purple martins LOVE these but you will need a lot.  They like living in condos.  Be sure to drill a few small drainage holes on the bottom, and a few vent holds on top, preferably under the upper bulge to help keep rain out. The extra holds help keep the birds from overheating.

3.  Egg shells prep.
Save your cracked egg shells.  And save the carton. It makes a great shell rack.  You'll need to remove a membrane from inside.  Holding a half shell under cold water while you rub the inside will usually work pretty easy.
Let dry.

Paint your shells with any acrylic paint.  I use cheap craft paints you can find just about anywhere.  Some paints may require a few coats, some won't.  The sky is the limit here.

Paint your gourd too.  

4.  Applying the shells.
You will need some white glue, and an exacto knife.
Break off a hunk of painted shell.  Say the size of a quarter.  Put a small drop of glue on your gourd.  Place a corner of the shell on the glue and with the tip of your knife, break off a chip of the shell right onto the glue.  With practice, you can eventually lay down a several inch bead of glue and work in small sections pretty fast.  
Keep the glue to a minimum, and don't worry much about glue showing.  It will dry clear. Less is more here.  And this will take some practice.

5.  Finally, when the whole thing is dry, you will want to seal it.  I've tried many products.  Clear acrylic sprays, poly urethane, it all depends on what you find easy to work with, and if this will be kept outside or not.  You really don't need to do anything to the inside although I have made a few planters where I cut the gourd right in half and hung the planter from chains.  Only because the inside is much more visible I painted it.  But for bird houses, or these decorative gourds, it's not necessary.

Now I didn't even go into designs.  That part is up to you!  Start simple.  Just make bands of color.  Or a rainbow.  Good easy way to get the hang of this before starting a complicated design.

HAVE FUN!