My advice will be like a broke person telling someone else how to get rich.
Cut a V in the root stock. Cut a wedge in the part you wish to graft on. I wouldn't have the new addition no more than about 6 inches long. If you have ever looked at a piece of fire wood that's been cut with a saw and laying on its side, you can see the difference in the outside bark and the rest of it. With the outside bark in mind - make sure the bark of each graft is touching the other bark. This is where the nutrients and such flow. If you don't have those two alligned together the graft most likely won't take.
Then you need to wrap it up tight, splint, and wrap some more. Keep some moisture there as well.