Answer:
Break of an "elbow" with sharp knife or pruners. Watch out for the drippy white stuff-- it is like the pointsettia flower. Let the plant dry out for about 2 weeks, out of the sun.
Some people use rooting hormone powder at this stage. I just put the piece of plumeria into sandy soil in a 6 inch pot and water just enough to settle the soil down. These plants are drought resistant and very hardy. DO NOT over water-- the stem rots easily.
You can put the pot directly into the ground. If you plant the plumeria into the ground it will have to stay there. They don' like to be moved. By putting it into pot, you can move it around your yard if necessary.
Good luck!