I have searched that material, but I could not find.
But that concept is very easy.
First you have to sample the wave about 32 or 64 with ADC and OPAMP (Do not use diode rectifier!!!)
Second, you may use RMS formula through DSP or high speed MCU.
(It is used to floating point math).
Third, you need to multiply the value by scale factor.
That is all.