Its quite a complicated procedure
It involves sourcing a fuel guage that fits your tank, and a compatible fuel meter. Then a good denter fits the guage in your tank, making sure he doesn't damage the tank's sheet metal, and that there are no future leaks from where the guage has been fitted. Next, the front meter assembly is redesigned to accommodate the new fuel meter. Lastly, a senior electrician joins all circuits
Sounds pretty tough......it sure is!
But you have an easy way out since you plan to do it on a cg125
Buy a honda deluxe tank(with functional fuel guage) and front meter assembly from scrap. .....The rest of the job would be quite easy then