A method for implementing a supporting base (1) of a car seat, the latter including said base apt to rest onto a seat of such vehicle and apt to support an upper structure (16) apt to receive and retain a child, the base comprising a first (2) and a second (3) portion, the first sitting portion (2) being apt to rest onto the seat sitting and the second backrest portion (3) being apt to rest onto the backrest of such seat. It is provided to implement such base in at least two sections (20, 21), each section (20, 21) comprising a portion of the base portion and a portion of the sitting portion of the base, said sections being then coupled therebetween and constrained so as to implement the above-mentioned base. The so-obtained base and the seat equipped with such base are also claimed.