Insert the SD card with PiDoctor into your Raspberry Pi, and boot it.
Monitor the ACT LED, it will blink irregularly first. PiDoctor will automatically start.
Once the ACT LED will start to blink slowly and regularly (2 sec on, 2 sec off), the tests are done. You can unplug the Pi from power (no need for shutting it down) and remove the SD card.
Hint: if you want to test multiple Pi's, simply repeat this step. An individual test result file will be created. Note the sequence in which you tested your Pi's.