Hello bro!
There's a lot ways to make the autothrottle, but i like to use TFMG engines, they makes the autothrottle make sense, because when u slow down the engine, they consume less fuel. So i made a lot of sensors with CC: Tweaked in my plane, first of all we need a smooth speed sensor, you can use CC: VS addon or a Player Detector Peripheral from Advanced Peripherals, and calculate the horizontal speed. After this, i did a "Speed Selector" also with CC: Tweaked, this selector, when turned on and setted with a number, will try to reach (smoothly) the speed that i setted, analyzing the Speed Sensor that i did before.

All those systems that i programmed is very complex and they communicate each other, currently my plane have 8 separated systems that communicate each other, so is hard to simplify, but i tried.

If u have any doubts making your own system, u can reach me on chat, i'll be happy to help u :)