By infusing the energy threads, which he can't control, with something like his internal energy, which he can control, he can manipulate the energy threads somewhat and they become controllable.
However, getting an energy thread to swell is a very difficult process. In most cases, all of the energy in one's energy chamber won't be enough to accomplish this. So, rank 3 cultivators usually need to supplement their internal energy reserves with an external source of energy, which is what energy stones are.
Fortunately, he has four other energy chambers that can assist a single energy chamber, so he can make an energy thread quickly become swollen without spending money.
Once he can move the energy, the thread becomes swollen the rest becomes somewhat easy. All he needs to do next is to press the swollen energy thread onto the budding energy nexus. Over time, the swollen energy thread will fuse with the budding energy nexus.
