Combine the egg whites and powdered sugar in a medium-size mixing bowl and whip with an electric mixer on medium speed until opaque and shiny, about 5 minutes. Add the lemon juice and continue whipping until completely incorporated, about 3 minutes. The lemon juice whitens the royal icing. The royal icing should be light, fluffy, and slightly stiff. You may need to adjust the consistency by adding more egg whites if the icing is too dry or more powdered sugar if it is too wet.
Make a cornet and fill it half full with royal icing. Cover the remaining icing with a damp cloth to keep it from drying out while you work. It will be much easier to decorate the cake if you place it on a turntable. Use the royal icing to make decorations as you desire. Make as many cornets as needed to complete the design. You can use a cornet filled with royal icing to fill in the seams and edges where the cakes meet. Decorate with dots, swags, or any other design. The royal icing will harden as it dries.
Piping fine decorations with royal icing takes some practice but you will improve with time. To train your hands and practice piping, try this exercise: use a medium-point magic marker to draw the desired pattern on a sheet of parchment paper. Repeat the pattern several times to allow for practice. Place another sheet of parchment paper over the designs and tape both pieces to the work surface to keep them from moving. Use a cornet filled with royal icing to trace the pattern. When you feel you have had enough practice, place the cake in front of you and repeat the patterns on the cake, instead of on the parchment paper.
For different widths of decoration, use several cornets and cut the ends into different-sized openings. With royal icing, if you make a mistake, you can just wipe it off! The royal icing will harden as it dries.
Flowers are widely used to decorate cakes. Usually, when the flowers are not fresh, they are made from gum paste. You can make your own or buy them already made from a specialty baking supply store. I would suggest buying them since they are very reasonably priced compared to the time and energy it takes to make them. Gum paste flowers can be reused or stored in an airtight container for any celebration. Remember to handle them with care because they will chip or break.
If you have your own garden, pick some lovely blooms and use them to decorate the cake. Don't forget to use the natural greens to decorate around the base of the cake. Be sure you know which varieties of flowers and greens are safe to use; there are some that are poisonous. Flowers that are safe to use include: apple blossoms, citrus blossoms, day lilies, English daisies, pansies, roses, tulips, lilacs, and violets.
Consumption of raw or undercooked eggs, shellfish and meat may increase the risk of foodborne illness.