The 10–20 system or International 10–20 system is an internationally recognized method to describe and apply the location of scalp electrodes in the context of an EEG test or experiment as well as tDCS. This system is based on the relationship between the location of an electrode and the underlying area of cerebral cortex. The “10” and “20” refer to the fact that the actual distances between adjacent electrodes are either 10% or 20% of the total front–back or right–left distance of the skull. (Source: Wikipedia)
There were modifications made to the 10-20 system that expanded it to what is called the 10-10 system. For the purposes of tDCS, we use a slighly expanded version of the 10-20 system.
We are currently designing a manual on how to make those measurements. In the meantime, there are lots of resources online. Like this YouTube video.