Train in your nen and the aura will naturally grow. First, you need to find a nen master who can train you to open and close your pores to release/stop aura flow. Meditation helps too as you learn to control your aura output. Once you have a good grip on nen, there is a way to focus the nen on your eyes so you can spot people’s otherwise-hidden nen abilities. There is also a way to become “invisible” to detect where you completely close off the flow of aura and a way to make a special ability.