Calculating FIR filter coefficients with the windowing method and Convolution algorithm
draw your desired frequency response.  uniformly sample that frequency response $\frac{N}{2}$ times from DC up to just below Nyquist.  reflect those $\frac{N}{2}-1$ samples (not the DC values) as complex conjugate in the upper half of the frequency response above Nyquist.  do the inverse FFT.  if it's rectangular window, you're done.