Summarize:

Mix 1 part decoupage glue and 1 part water in a small bowl. Brush the mixture onto the outside of the eggs with a small paintbrush, then use an eyedropper to squeeze 2 to 3 drops into one of the holes on the egg. Close both holes with your fingers and shake the liquid to coat the inside of the egg. Let the eggs dry completely.  You can buy decoupage glue from a craft store or an online retailer. Place a towel under the eggs while they dry since some remaining liquid may come out. Squeeze acrylic puff paint onto the eggshell to create 3D patterns like stripes, swirls, or dots. Let the paint dry for 2 to 3 hours.  You can also make a design with puff paint, then paint over it with acrylic paint once the puff paint has dried. This creates a cool raised effect underneath. To prevent the paint from smearing while it dries, slide a bamboo skewer through the holes. Stand the skewer upright or lay it over an empty container so the egg is suspended in the air. You can find washi tape in all different patterns, colors, and textures at a craft store. Wrap 2 to 3 strips of the tape around an egg if you want to leave some of the shell showing, or cover the egg completely with tape for an all-over decoration.  If you want an artsy mosaic look, cut out small pieces of washi tape and arrange them around the egg, leaving white space in between each piece. You can also paint the egg with acrylic paint first if you want a more colorful craft. Let the paint dry completely for 2 to 3 hours before you add your washi tape on top.
Apply decoupage glue and water over the eggs to make them last longer. Draw designs with puff paint if you want eggs with texture. Use washi tape to add colorful accents to the eggshells.