Put the severed cuttings on a clean piece of paper and place them in a partial shady place at a minimum temperature of 10 Celsius degree (50 F) and allow 24-48 hours for the wound to callus.
Prepare the pots by filling them almost to the brim with equal parts of fine peat substitute and sharp grit or sand. Insert the cuttings so that the base is just held in the mixed soil. Firm then the soil around the cuttings. Cover the surface with a top-dress of gravel or grit to help the cuttings to stay in the position.
Label and place the pots in dappled shade at a constant temperature at about 21 Celsius degree (70 F). Keep the cuttings moist by watering them every day with tepid water. Use a fine mist-spray to not disturb the cuttings too much.
Rooting will usually occur fast and in about two weeks new growth will appear. After the new growth will develop you can repot the plantlets into a new pot filled with loam-base potting compost.
Some people use Crassula for bonsai arrangements. You could try too because is an easy plant to grow.