There’s many ways. But the most common is a single graft on a young JBP seedling. Once the graft takes, the top of the JBP portion is cut off. Eventually, the two parts grow together, so it had JBP roots and lower trunk, then the upper portion and all the foliage will be JWP.
sometimes, several grafts can be placed on the JBP trunk. Lower grafts become branches, and the top graft becomes the rest of the tree.
you don’t have to start with a you g JBP seedling. It’s possible to graft onto any trunk, although as the JBP trunk gets older, the bark gets thicker, and it’s more difficult to get the cambium layers aligned just right.