Things you need to know. The sampling rate. The fundamental frequency which you wish to isolate. FIR filter basic and what order do you want. I would suggest making a band stop filter, with a thin stop band. stop band frequencies fc1 and fc2. Then use the code below.
fc1 = fundamental_freqeuncy1/sampling_rate;
fc2 = fundamental_freqeuncy2/sampling_rate;
N = 10; % number of taps you can change
n = -(N/2):(N/2); % 11 order filter
n = n+(n==0)*eps;
[h] = sin(n*2*pi*fc1)./(n*pi) - sin(n*2*pi*fc2)./(n*pi);
h(0) = 1 - (2*pi*(fc2-fc1))/pi;
%coefficient for band stop filter... which could be used % as notch filter
[w] = 0.54 + 0.46*cos(2*pi*n/N); % window for betterment
d = h.*w; % better coefficients
Now d are the filter coefficients. and your filter is
freqz(d); % use this command to see frequency response
After this you need to filter your input using the filter designed by coefficients 'd'. so
y = filter(d,1,X) % X is input, y is filtered output