Plum tree cuttings help?

What is most important in the process to allow roots to grow on a cutting, is the cutting itself. I would cut off a shorter piece of the branch, one that doesn't have thick bark on it, and continue with the process as you mentioned. It is almost impossible to get a branch that has old bark on it to root. Wait till the tree has put forth new growth and the bark is very thin and possibly still green; you will have better luck getting it to root.