To propagate a branch from a Northern Catalpa tree, you can use a method called layering. This involves bending a branch down to the ground and securing it in place with a weight or a stake. The branch should be bent so that it touches the ground without breaking. Once the branch is in place, you can cover it with soil or a potting mix to encourage root growth. Keep the soil moist and after a few weeks, you can cut the branch from the parent tree, leaving a few inches of the stem attached to the new roots. Then, you can plant the new rooted branch into a pot or directly into the ground. It's important to keep the soil moist during this process to encourage root growth.