Hi
Plumbago is quite easy to propagate. Take cuttings of about an index finger or an index finger and a half. Cut them just below a join. Snip off all the leaves except for one or two at the top end of the cutting (it is best to cut the leaves off rather than tear them off as this does not damage the stem). Use a sharp pruning shears , kitchen scissors or knife when taking the cuttings as it is best to get a nice clean cut rather than a ragged one so as to minimise the chances of fungal rot setting in. It is also best to take semi hardwood cuttings ie. last years growth. Put the cuttings in clean water to soak for about 15 minutes. Remove the cuttings and if you have some hormone powder or liquid (eg. Dip and Grow etc.) dip them in the hormone agent as per the manufacturers instructions.
You will need a well drained plastic container (a plastic ice cream tub works well but be sure to make sufficient drainage holes). Fill the container with sharp river sand . Take a stick and make holes for the cuttings to be placed into in the riversand filled container (be carefull to put them in the right way up as they will die if upside down). Firm the sand around each cutting with your fingers. Water the container and right any cuttings which may have become dislodged by the water. Place the container in a shaded area. Cuttings must never be allowed to dry out so check that the sand is still moist every day. They should take about 4 to 5 weeks to form roots. You should notice new new little leaves coming out. When they look as if they are ready immerse the whole container in a bucket of water , stick you fingers into the sand a bit away from the cutting and go right down to the bottom of the container , scoop the cutting with its surrounding sand up into your hand and agitate a bit so that the sand comes away in the water. If it is right you should see little roots have formed. If no roots have yet formed remove the container from the water, make holes in the sand and gently insert them back into the sand and put them back into their shady spot.
If they have roots plant them in a mixture of half garden sand and half compost , preferrably in a pot or nursery plastic bag. Keep them in morning only sun and moist until they are nicely established and a reasonable size then plant them out into your garden into a full sun spot.
Good luck. Garrick