It's pretty simple to use - put the HT radio on the aircraft band on a quiet channel. It should be in AM mode. Back off the squelch so you can hear static, then adjust the volume to a comfortable level and start pointing around. If you point in various directions and hear more noise, you're on the way.
Rene, N6NRN was the first to use this. He had bad noise at his home QTH, and was able to locate a noisy pole right near his house. The electric company (PG&E) was actually very responsive, and came out and fixed the problem.
We have an attenuator unit that can be useful with different noise levels, but it's a bit heavy and sometimes not needed.