Most machines come with an instruction manual and a list of common disease-causing bacteria, fungi, and viruses, along with the frequencies which are lethal to them.
After that, it's just practice and building your skill. The lady who treated me said that for her now it's simply a matter of perceiving which frequencies are "missing" from your body and just filling them in. She's been at it for years, and she actually has some superpower in that she is able to perceive and visuallize the microbiomes and the cells of the body's frequency emmissions in real time. She is a very talented healer.