how to make car back windows go down?

Probably not possible. The EF Civic 3dr has rather long rear side windows, and a low waistline. Even if you could modify the body side inner and outer panels to create a gap between them at the window sill, there isn't enough height between the waistline and the wheelhouses (and the fuel filler) for the glass to drop more than an inch or two.

To see how challenging making rear drop-glass work is, look at the current Mercedes E-Class coupé which has rear drop-glass, but with a small fixed quarterlight behind the drop-glass because there isn't space in the rear panel to allow the whole rear window to drop.A pop-out rear side glass, which many 2-door and 3-door cars had from the 60's to the early 90's, is more feasible. Looking online at images, some EF Civics had this. It is now even used on some 5-door cars to save development costs and add design freedom (Citroën C1, C4 Cactus, DS4,Toyota Aygo).