How can I create simple low pass filter for filtering out the higher frequency from the resulting output?

Hi, I am trying to remove the carrier of signal by multiplying it with another signal of same frequency as the carrier frequency of the first signal.
After multiplication I need to filter out the higher frequency element and retain the lower frequency element, for which I must use a low pass filter.
How can I implement this LPF without having signal processing toolbox available with me? Any lead would be appreciated.
Thanks, Priyanka

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