How to Make It
Knead together gum paste and shortening on a clean work surface until shortening is fully incorporated and gum paste is soft and malleable. (It may be easier to work with half of the gum paste and shortening at a time.) Wrap half of the gum paste mixture tightly in plastic wrap, and set aside.
To make petals: Use a rolling pin to roll the unwrapped half of the gum paste mixture to 1/8-inch thickness. Cut 12 circles out of the gum paste mixture using a 1-inch round cutter. Cut 24 circles using a 2-inch round cutter, rerolling scraps as needed. (To prevent gum paste mixture from drying or cracking, work quickly and keep it covered with plastic wrap or a damp paper towel as you go.)
Use small scissors to cut each circle to create a light bulb shape.
Place each petal on a foam mat, and (using the rounded bottom of a measuring teaspoon) gently and gradually smooth the edges of the petal until they are slightly thinner. Place 3 or 4 petals in bottom and against side of a small bowl, shaping each petal to the curvature of the bowl. (Do not overlap petals.) Repeat with the remaining petals. Let petals stand in the bowls, uncovered, overnight.
To make stamens: Unwrap remaining half of gum paste mixture. Pinch off 4 pea-size balls, and rewrap remaining gum paste mixture. Roll each ball into a cone shape. Flatten bottom of wide end of each cone, and stand upright on a work surface. Using small scissors, cut little incisions at a downward angle all over sides of cone. Let stand, uncovered, overnight.
To make leaves: Divide reserved gum paste mixture from Step 4 into 2 equal portions. Wearing gloves, knead 8 drops of green food coloring into 1 portion of gum paste mixture until thoroughly blended, kneading in 1 more drop at a time until color resembles green side of a magnolia leaf. Cover with plastic wrap. Knead 8 drops of brown food coloring into remaining portion of the gum paste mixture until thoroughly blended, kneading in 1 more drop at a time until color resembles brown side of a magnolia leaf. Roll each green and brown portion into 1 (1/8-inch-thick) sheet. Place the green sheet on top of the brown sheet, and roll together to 1/8-inch thickness.
Cut out about 15 leaves using a leaf-shaped cutter. (Or, using a 2-inch round cutter, cut into about 15 circles, and, with small scissors, trim circles into leaf shapes.) Place each leaf on foam mat, and (using rounded bottom of a measuring teaspoon) gently and gradually smooth edges of leaves until they are slightly thinner.
Using dull side of a paring knife, create a vein pattern on the leaves. (If using a leaf-shaped mold, press each leaf into mold, and gently remove.) Place 2 of the wooden spoons side by side, with handles as close as possible. Drape leaves over handles, with center vein of leaf between and parallel to handles. (Do not let leaves touch each other.) Repeat with remaining leaves and spoons. Let stand, uncovered, overnight. (This will give the leaves a natural curve.)
To make flowers: Place a 1-inch-square piece of paper towel in center of a small bowl. Arrange 3 large petals in bowl, allowing the narrow ends to overlap in the bottom center of the bowl. Dab a generous amount of edible glue in between each petal end, and gently press to adhere. Place 3 large petals on top, arranging them in gaps of petals already in bowl. Dab a generous amount of edible glue in and around the center of the petals, and press to adhere. Place a small ball of aluminum foil behind each petal to keep layers of petals from touching. Arrange 3 small petals on top of gaps of petals in bowl. Dab a generous amount of edible glue in and around the center of the petals. Press to adhere. Place a small ball of aluminum foil behind each petal to keep layers of petals from touching. Repeat with remaining large and small petals in 3 separate bowls to form 3 more flowers.
Using a food-safe paintbrush, brush 1 stamen with a mixture of green and yellow luster dust; blow off excess.
Dab a small amount of edible glue on flat end of stamen, and press into center of 1 flower. Place small aluminum foil balls around the stamen to hold upright. Repeat with remaining stamens and flowers. Let stand, uncovered, at least 24 hours or up to 48 hours.
Once the glue has dried, remove and discard the aluminum foil balls from between petals and from around stamens; remove and discard paper towel squares. Place magnolia flowers on top of frosted cake, pressing lightly to secure. Arrange leaves around each flower or along bottom of cake, forming a garland. (The flowers will keep in an airtight container in a cool, dry place up to 4 months.)