The crown-of-thorns plant (euphorbia milii) can be propagated by taking a cutting from some of the most recent woody growth and rooting it. This process of rooting the cutting is not complicated if you have the right materials and some time and patience. Within a few months, you will have a new crown-of-thorns plant growing in your window producing cheerful blossoms almost all year long.
Look on your crown-of-thorns plant for a healthy branch with lots of leaves on it. Cut a 3- to 4-inch section at a 45 degree angle across the stem with your pruning shears, being careful of the thorns. Immediately place the cutting into a cup of water. Wear gloves to protect yourself from the thorns and the latex sap (a skin irritant).
Allow the clipping to sit in the cup of water until the milky sap has stopped flowing. If you are not sure, pour out the water and refill the cup to get a better view. This sap is very irritating, so wash your hands with warm soapy water.
Set the stem into some rooting hormone powder and let it sit there for 24 hours. Have a plant pot ready with a soil mixture of equal parts of sand, perlite, and peat moss. Moisten the soil and allow it to drain while waiting during the 24-hour period.
Place the crown-of-thorns cutting into the prepared pot of potting soil. Press the soil up against the stem to make sure it has good contact. Set the plant in a warm place (70-75 degrees) where it will get several hours of sunlight each day.
Water the soil around the plant (but not the leaves) when the soil feels dry to the touch on top. Make sure the pot does not sit in the drained water. Within a few weeks, you should see signs of new growth.