How to apply butterworth filter to downsampled data?

I have a function that downsamples and normalizes data:

function DATA = dylan(filename,n)
% Downsample and Normalize
%   Downsample to Hz required (n), Normalize
f = filename;
k = downsample(f,n);
DATA = normalize(k);
save('DOWNEDDATA.mat','DATA');
end 

And I need to filter this downsampled data from a range of 0.1 to 200 Hz. I was thinking of employing a butterworth filter for this task, but am confused on how to specify the output downsampled data variable I want to actually filter in the butterworth filter. How would I go about filtering this product data I have downsampled (0.1 - 200 Hz)? My data is at 1000 Hz downsampled right now.

Thank you.