3D to 2D? simplest way?

3D to 2D? simplest way?

I have question: how to simple "convert" 3D to 2D? I actualy play with
microcontrollers(AVR) and some time ago connect LCD from old phone. I want
to render some simples(cube), i have formulas to rotate, move, etc... BUT
i cant display it all cos cant covert 3D(wich action happens in RAM) to 2D
simple points on screen wich i connect with line. ive tryed some ways i
found on internet: x'=-z*sin(a)+x*cos(a)
y'=-(z*cos(a)+x*sin(a))*sin(b)+y*Cos(b) or some like that, but it ot seems
to work.
P.S. Dont tell me some like: Google is your friend, etc.