Place the device on the dorsolateral prefrontal cortex, which is used for thinking and memory. If you are right-handed, place the Thinkie sensor above the left eyebrow, slightly to the left. For left-handed people, it is said that half the time the left side of the brain works better and the right side works better the other half. If you are left-handed, you can experiment with what works best for you. Try it first on the left side, and then on the right side. After determining which side provides the most brain activity, place the device where it will be more active when you do your brain training.
Be careful to clear all hair from under the Thinkie sensor. If strands of hair are caught between the sensor and your forehead, it will not be able to measure correctly.