Correct way of zero padding in time domain
Zero padding changes the bin size of the DFT resulting a finer frequency resolution. It is done by appending zeros to the end of the signal because we are only artificially increasing the length to decrease the bin size, $\frac{2\pi k}{N}$. An $N$ point DFT is just evaluating the DTFT on certain frequencies. If you were to append some zeros to the front of the signal then you are doing a time shift, and a shift in time domain is modulation in the frequency domain: $x[n-k]=X(\omega)e^{-j\omega k}$.