This is the most difficult and important part. The sound hole should be placed near the wall, but not right at the edge and the size of the opening will affect the quality of sound. The opening should be squared precisely with the windway, neither larger nor smaller. If the window is too large or too small, it will not whistle properly. Start with a rectangle a bit less than 1/4 inch long and enlarge later as needed.
Use the mouthpiece you made to position where the voicing should go, and draw on a matching sound hole area to leave empty of plastic.
If it is too small, it will sound weak or not at all with holes covered. If the angle of the labium (where the air hits to produce sound) isn't acute enough the sound will also be weak or easily overblown on low notes. The labium needs to taper almost to a point, and be at a 45 degree angle to the windway.
Doodle over the whole thing just like you did in Step 1 using ABS plastic, adding extra plastic to form the labium. Also add side walls to help direct the wind into the labium. Using the edge of the nozzle and/or the smoothing tool to shape the labium. Just get the general shape right now, you’ll finish it later.
Doodle another layer of plastic with your chosen colors just like you did with the front of the Ocarina, leaving space around the voicing, labium, and sidewalls.
Remove the Doodle from the form.
Use an x-acto knife to carve the sound hole square and carve the labium to be smooth and at a 45 degree angle to where you will add the mouthpiece. The airstream needs to be directed through the mouthpiece directly at the beveled labium so that it cuts the air evenly in two. Carefully trim the lip to a blunt edge on the outside of the bottom dome, and bevel it a tiny bit on the inside as well. You can smooth it out with the smoothing tool on the 3Doodler as well.
Attach the mouthpiece very carefully by adding a small amount of plastic to connect the mouthpiece to the back of the Ocarina. Be careful to not get excess plastic in the windway and gently carve any away with the knife. Add more plastic until it stays in place. Test to make sure it is aligned properly. Remember the 45 degree angle. Look into the mouthpiece with the convex part of the dome part on top, the labium edge should be visible at about halfway through the mouthpiece, a bit lower is okay too.
Make sure it whistles by cupping the convex part of the dome gently in your palm and blowing into the mouthpiece. If it sounds good, finish attaching the mouthpiece. Finish up the sidewalls, making them match the mouthpiece.
Add more plastic to make sure the mouthpiece is nice and square, being careful not to get any in the windway.
Touch up the interior, adding plastic to make it smooth, and using the smoothing tool on the 3Doodler as well to be sure. Carve the area surrounding the soundhole to make sure it is slightly beveled.