Choose a stem from the current year's growth that has passed the pliable stage (easy to bend) but has not yet reached the hard, woody stage (impossible to bend). You are looking for a stem that snaps when you bend it. This is known as a softwood cutting. The portion of stem that you choose should have at least two leaf nodes (where the leaves join the stem).
Cut a 6-inch length of stem from the tree, 1 inch below a leaf node Wrap the cutting in moist paper towels and place it in a plastic bag until you are ready to plant it. Keep it out of direct sun so that it doesn't dry out.
Pour equal parts of perlite and peat moss into a small pot and water it until the water drains from the bottom of the pot. Poke a planting hole in the medium with a pencil or your finger.
Strip the mock orange cutting of its leaves, leaving two leaves at the top of the cutting.
Dip the bottom inch of the cut end of the stem into the rooting hormone and tap the cutting on the side of the jar to remove any excess hormone.
Stick the cut end of the mock orange stem into the prepared planting hole and pack the soil around it.
Water the cutting again, to settle the soil around the cutting. Allow the pot to drain for at least 10 minutes.
Poke wooden skewers into the soil around the perimeter of the pot. Place the pot into the plastic bag, adjusting the plastic so that the skewers hold it away from the leaves. Seal the bag and place the pot in an area that is free of drafts and out of direct sun.
Check the soil periodically to ensure that it remains moist, but not soggy.
Check for root development in four to five weeks by looking for small white roots at the drainage holes in the bottom of the pot.
Repot the rooted cutting into a one-quart pot filled with a mixture of 80 percent potting mix and 20 percent perlite. Water it until the water drains and keep the soil moist. Place the pot in a sunny area.
Fertilize the new double mock orange tree with seaweed or kelp-based fertilizer three weeks after repotting.
Bring the plant indoors for its first winter and transplant the double mock orange tree into the garden in spring.