If you want to write code for demodulation without using amdemod, then you need to first select which method are you going to use for demodulation: Envelope Detector , Square law demodulator etc.
Assuming you select Square law Detector, you have a AM equation of form
V(t) = A(1+km(t))cos(2*pi*fc*t);
need to mathematically model the behaviour of square law device into equations of form
Vsq(t)= k1V(t) + k1(V(t)*V(T)) + ...
then you need to mathematically model a low pass filter and pass the signal through it(model this behaviour either by convolution or convert to frequency domain and multiply both filter response and Square law output and then inverse fourier to get time domian signal) to get AM output.