Lightly grease a clean work surface with shortening, and knead the gum paste with the palm of your hand against your work surface until pliable.
Take a 6" length of floral wire, and use your needle nosed pliers to make a small, sharp hook on one end. Dip the wire into a bit of water, and insert it (hook side in), at the base of one of your cut rose leaves, being careful to guide it into the gum paste without cutting it open.
(If your gum paste is too thin, it will break through, and you may need to re-roll and cut the leaf shapes. If the wire shows on the back of the leaf a little bit, don't worry about it; once the leaves are colored, this will not matter.)
Using a tea ball or cheesecloth bag, lightly dust your silicone leaf veiner with cornstarch, and press the leaf between the two pieces firmly. Remove the top piece, and gently remove the leaf.