How to apply an anti-aliasing filter before downsampling
Anti-aliasing filtering is applied just as any other LTI filtering: If your input data is $x[n]$, and the impulse response is $h[n]$, then your output will be
$$y[n] = x[n] \star h[n] $$
where $\star$ is the convolution operation, a.k.a. the anti-aliasing filtering in this context.
Your impulse response $h[n]$, ideally, corresponds to a lowpass brickwall filter, with a cutoff frequency of
$$ \omega_c = \frac{\pi}{M} $$ radians per sample, where $M=5$ is the downsampling ratio in your particular application. You may design $h[n]$ by any suitable method, but for maintaining high accuracy, you better use tight specs on the desiged filter, this can be achieved by a sufficiently long FIR linear-phase filter, or similar IIR filter. The latter will be more efficient to implement but less accurate due to its nonlinear phase.
Then you can decimate $y[n]$ by $M$ as $$v[n]= y[Mn]$$
to  get the downsampled sequence.